WordPress Plugin Development (Core)

Last Updated: 2026-01-21 Latest Versions: WordPress 6.9+ (Dec 2, 2025), PHP 8.0+ recommended, PHP 8.5 compatible Dependencies: None (WordPress 5.9+, PHP 7.4+ minimum)

Quick Start

Architecture Patterns: Simple (functions only, <5 functions) | OOP (medium plugins) | PSR-4 (modern/large, recommended 2025+)

Plugin Header (only Plugin Name required):

php

<?php
/**
 * Plugin Name: My Plugin
 * Version: 1.0.0
 * Requires at least: 5.9
 * Requires PHP: 7.4
 * Text Domain: my-plugin
 */

if ( ! defined( 'ABSPATH' ) ) exit;

Security Foundation (5 essentials before writing functionality):

php

// 1. Unique Prefix
define( 'MYPL_VERSION', '1.0.0' );
function mypl_init() { /* code */ }
add_action( 'init', 'mypl_init' );

// 2. ABSPATH Check (every PHP file)
if ( ! defined( 'ABSPATH' ) ) exit;

// 3. Nonces
wp_nonce_field( 'mypl_action', 'mypl_nonce' );
wp_verify_nonce( $_POST['mypl_nonce'], 'mypl_action' );

// 4. Sanitize Input, Escape Output
$clean = sanitize_text_field( $_POST['input'] );
echo esc_html( $output );

// 5. Prepared Statements
global $wpdb;
$wpdb->get_results( $wpdb->prepare( "SELECT * FROM {$wpdb->prefix}table WHERE id = %d", $id ) );

Security Foundation (Detailed)

Unique Prefix (4-5 chars minimum)

Apply to: functions, classes, constants, options, transients, meta keys. Avoid:

wp_

__

php

function mypl_function() {}  // ✅
class MyPL_Class {}          // ✅
function init() {}           // ❌ Will conflict

Capabilities Check (Not is_admin())

php

// ❌ WRONG - Security hole
if ( is_admin() ) { /* delete data */ }

// ✅ CORRECT
if ( current_user_can( 'manage_options' ) ) { /* delete data */ }

Common:

manage_options

(Admin),

edit_posts

(Editor/Author),

read

(Subscriber)

Security Trinity (Input → Processing → Output)

php

// Sanitize INPUT
$name = sanitize_text_field( $_POST['name'] );
$email = sanitize_email( $_POST['email'] );
$html = wp_kses_post( $_POST['content'] );  // Allow safe HTML
$ids = array_map( 'absint', $_POST['ids'] );

// Validate LOGIC
if ( ! is_email( $email ) ) wp_die( 'Invalid' );

// Escape OUTPUT
echo esc_html( $name );
echo '<a href="' . esc_url( $url ) . '">';
echo '<div class="' . esc_attr( $class ) . '">';

Nonces (CSRF Protection)

php

// Form
<?php wp_nonce_field( 'mypl_action', 'mypl_nonce' ); ?>
if ( ! wp_verify_nonce( $_POST['mypl_nonce'], 'mypl_action' ) ) wp_die( 'Failed' );

// AJAX
check_ajax_referer( 'mypl-ajax-nonce', 'nonce' );
wp_localize_script( 'mypl-script', 'mypl_ajax_object', array(
    'ajaxurl' => admin_url( 'admin-ajax.php' ),
    'nonce'   => wp_create_nonce( 'mypl-ajax-nonce' ),
) );

Prepared Statements

php

// ❌ SQL Injection
$wpdb->get_results( "SELECT * FROM table WHERE id = {$_GET['id']}" );

// ✅ Prepared (%s=String, %d=Integer, %f=Float)
$wpdb->get_results( $wpdb->prepare( "SELECT * FROM {$wpdb->prefix}table WHERE id = %d", $_GET['id'] ) );

// LIKE Queries
$search = '%' . $wpdb->esc_like( $term ) . '%';
$wpdb->get_results( $wpdb->prepare( "... WHERE title LIKE %s", $search ) );

Critical Rules

Always Do

✅ Use unique prefix (4-5 chars) for all global code (functions, classes, options, transients) ✅ Add ABSPATH check to every PHP file:

if ( ! defined( 'ABSPATH' ) ) exit;

✅ Check capabilities (

current_user_can()

) not just

is_admin()

✅ Verify nonces for all forms and AJAX requests ✅ Use $wpdb->prepare() for all database queries with user input ✅ Sanitize input with

sanitize_*()

functions before saving ✅ Escape output with

esc_*()

functions before displaying ✅ Flush rewrite rules on activation when registering custom post types ✅ Use uninstall.php for permanent cleanup (not deactivation hook) ✅ Follow WordPress Coding Standards (tabs for indentation, Yoda conditions)

Never Do

❌ Never use extract() - Creates security vulnerabilities ❌ Never trust $_POST/$_GET without sanitization ❌ Never concatenate user input into SQL - Always use prepare() ❌ Never use
is_admin()
alone for permission checks ❌ Never output unsanitized data - Always escape ❌ Never use generic function/class names - Always prefix ❌ Never use short PHP tags

<?

<?=

- Use

<?php

only ❌ Never delete user data on deactivation - Only on uninstall ❌ Never register uninstall hook repeatedly - Only once on activation ❌ Never use
register_uninstall_hook()
in main flow - Use uninstall.php instead

Known Issues Prevention

This skill prevents 29 documented issues:

Issue #1: SQL Injection

Error: Database compromised via unescaped user input Source: https://patchstack.com/articles/sql-injection/ (15% of all vulnerabilities) Why It Happens: Direct concatenation of user input into SQL queries Prevention: Always use

$wpdb->prepare()

with placeholders

php

// VULNERABLE
$wpdb->query( "DELETE FROM {$wpdb->prefix}table WHERE id = {$_GET['id']}" );

// SECURE
$wpdb->query( $wpdb->prepare( "DELETE FROM {$wpdb->prefix}table WHERE id = %d", $_GET['id'] ) );

Issue #2: XSS (Cross-Site Scripting)

Error: Malicious JavaScript executed in user browsers Source: https://patchstack.com (35% of all vulnerabilities) Why It Happens: Outputting unsanitized user data to HTML Prevention: Always escape output with context-appropriate function

php

// VULNERABLE
echo $_POST['name'];
echo '<div class="' . $_POST['class'] . '">';

// SECURE
echo esc_html( $_POST['name'] );
echo '<div class="' . esc_attr( $_POST['class'] ) . '">';

Issue #3: CSRF (Cross-Site Request Forgery)

Error: Unauthorized actions performed on behalf of users Source: https://blog.nintechnet.com/25-wordpress-plugins-vulnerable-to-csrf-attacks/ Why It Happens: No verification that requests originated from your site Prevention: Use nonces with

wp_nonce_field()

and

wp_verify_nonce()

php

// VULNERABLE
if ( $_POST['action'] == 'delete' ) {
    delete_user( $_POST['user_id'] );
}

// SECURE
if ( ! wp_verify_nonce( $_POST['nonce'], 'mypl_delete_user' ) ) {
    wp_die( 'Security check failed' );
}
delete_user( absint( $_POST['user_id'] ) );

Issue #4: Missing Capability Checks

Error: Regular users can access admin functions Source: WordPress Security Review Guidelines Why It Happens: Using

is_admin()

instead of

current_user_can()

Prevention: Always check capabilities, not just admin context

php

// VULNERABLE
if ( is_admin() ) {
    // Any logged-in user can trigger this
}

// SECURE
if ( current_user_can( 'manage_options' ) ) {
    // Only administrators can trigger this
}

Issue #5: Direct File Access

Error: PHP files executed outside WordPress context Source: WordPress Plugin Handbook Why It Happens: No ABSPATH check at top of file Prevention: Add ABSPATH check to every PHP file

php

// Add to top of EVERY PHP file
if ( ! defined( 'ABSPATH' ) ) {
    exit;
}

Issue #6: Prefix Collision

Error: Functions/classes conflict with other plugins Source: WordPress Coding Standards Why It Happens: Generic names without unique prefix Prevention: Use 4-5 character prefix on ALL global code

php

// CAUSES CONFLICTS
function init() {}
class Settings {}
add_option( 'api_key', $value );

// SAFE
function mypl_init() {}
class MyPL_Settings {}
add_option( 'mypl_api_key', $value );

Issue #7: Rewrite Rules Not Flushed (and Performance)

Error: Custom post types return 404 errors, or database overload from repeated flushing Source: WordPress Plugin Handbook, Permalink Manager Pro Why It Happens: Forgot to flush rewrite rules after registering CPT, OR calling flush on every page load Prevention: Flush ONLY on activation/deactivation, NEVER on every page load

php

// ✅ CORRECT - Only flush on activation
function mypl_activate() {
    mypl_register_cpt();
    flush_rewrite_rules();
}
register_activation_hook( __FILE__, 'mypl_activate' );

function mypl_deactivate() {
    flush_rewrite_rules();
}
register_deactivation_hook( __FILE__, 'mypl_deactivate' );

// ❌ WRONG - Causes database overload on EVERY page load
add_action( 'init', 'mypl_register_cpt' );
add_action( 'init', 'flush_rewrite_rules' );  // BAD! Performance killer!

// ❌ WRONG - In functions.php
function mypl_register_cpt() {
    register_post_type( 'book', ... );
    flush_rewrite_rules();  // BAD! Runs every time
}

User-Facing Fix: If CPT shows 404, manually flush by going to Settings → Permalinks → Save Changes.

Issue #8: Transients Not Cleaned

Error: Database accumulates expired transients Source: WordPress Transients API Documentation Why It Happens: No cleanup on uninstall Prevention: Delete transients in uninstall.php

php

// uninstall.php
if ( ! defined( 'WP_UNINSTALL_PLUGIN' ) ) {
    exit;
}

global $wpdb;
$wpdb->query( "DELETE FROM {$wpdb->options} WHERE option_name LIKE '_transient_mypl_%'" );
$wpdb->query( "DELETE FROM {$wpdb->options} WHERE option_name LIKE '_transient_timeout_mypl_%'" );

Issue #9: Scripts Loaded Everywhere

Error: Performance degraded by unnecessary asset loading Source: WordPress Performance Best Practices Why It Happens: Enqueuing scripts/styles without conditional checks Prevention: Only load assets where needed

php

// BAD - Loads on every page
add_action( 'wp_enqueue_scripts', function() {
    wp_enqueue_script( 'mypl-script', $url );
} );

// GOOD - Only loads on specific page
add_action( 'wp_enqueue_scripts', function() {
    if ( is_page( 'my-page' ) ) {
        wp_enqueue_script( 'mypl-script', $url, array( 'jquery' ), '1.0', true );
    }
} );

Issue #10: Missing Sanitization on Save

Error: Malicious data stored in database Source: WordPress Data Validation Why It Happens: Saving $_POST data without sanitization Prevention: Always sanitize before saving

php

// VULNERABLE
update_option( 'mypl_setting', $_POST['value'] );

// SECURE
update_option( 'mypl_setting', sanitize_text_field( $_POST['value'] ) );

Issue #11: Incorrect LIKE Queries

Error: SQL syntax errors or injection vulnerabilities Source: WordPress $wpdb Documentation Why It Happens: LIKE wildcards not escaped properly Prevention: Use

$wpdb->esc_like()

php

// WRONG
$search = '%' . $term . '%';

// CORRECT
$search = '%' . $wpdb->esc_like( $term ) . '%';
$results = $wpdb->get_results( $wpdb->prepare( "... WHERE title LIKE %s", $search ) );

Issue #12: Using extract()

Error: Variable collision and security vulnerabilities Source: WordPress Coding Standards Why It Happens: extract() creates variables from array keys Prevention: Never use extract(), access array elements directly

php

// DANGEROUS
extract( $_POST );
// Now $any_array_key becomes a variable

// SAFE
$name = isset( $_POST['name'] ) ? sanitize_text_field( $_POST['name'] ) : '';

Issue #13: Missing Permission Callback in REST API

Error: Endpoints accessible to everyone, allowing unauthorized access or privilege escalation Source: WordPress REST API Handbook, Patchstack CVE Database Why It Happens: No

permission_callback

specified, or missing

show_in_index => false

for sensitive endpoints Prevention: Always add permission_callback AND hide sensitive endpoints from REST index

Real 2025-2026 Vulnerabilities:

All in One SEO (3M+ sites): Missing permission check allowed contributor-level users to view global AI access token
AI Engine Plugin (CVE-2025-11749, CVSS 9.8 Critical): Failed to set
```
show_in_index => false
```
, exposed bearer token in /wp-json/ index, full admin privileges granted to unauthenticated attackers
SureTriggers: Insufficient authorization checks exploited within 4 hours of disclosure
Worker for Elementor (CVE-2025-66144): Subscriber-level privileges could invoke restricted features

php

// ❌ VULNERABLE - Missing permission_callback (WordPress 5.5+ requires it!)
register_rest_route( 'myplugin/v1', '/data', array(
    'methods'  => 'GET',
    'callback' => 'my_callback',
) );

// ✅ SECURE - Basic protection
register_rest_route( 'myplugin/v1', '/data', array(
    'methods'             => 'GET',
    'callback'            => 'my_callback',
    'permission_callback' => function() {
        return current_user_can( 'edit_posts' );
    },
) );

// ✅ SECURE - Hide sensitive endpoints from REST index
register_rest_route( 'myplugin/v1', '/admin', array(
    'methods'             => 'POST',
    'callback'            => 'my_admin_callback',
    'permission_callback' => function() {
        return current_user_can( 'manage_options' );
    },
    'show_in_index'       => false,  // Don't expose in /wp-json/
) );

2025-2026 Statistics: 64,782 total vulnerabilities tracked, 333 new in one week, 236 remained unpatched. REST API auth issues represent significant percentage.

Issue #14: Uninstall Hook Registered Repeatedly

Error: Option written on every page load Source: WordPress Plugin Handbook Why It Happens: register_uninstall_hook() called in main flow Prevention: Use uninstall.php file instead

php

// BAD - Runs on every page load
register_uninstall_hook( __FILE__, 'mypl_uninstall' );

// GOOD - Use uninstall.php file (preferred method)
// Create uninstall.php in plugin root

Issue #15: Data Deleted on Deactivation

Error: Users lose data when temporarily disabling plugin Source: WordPress Plugin Development Best Practices Why It Happens: Confusion about deactivation vs uninstall Prevention: Only delete data in uninstall.php, never on deactivation

php

// WRONG - Deletes user data on deactivation
register_deactivation_hook( __FILE__, function() {
    delete_option( 'mypl_user_settings' );
} );

// CORRECT - Only clear temporary data on deactivation
register_deactivation_hook( __FILE__, function() {
    delete_transient( 'mypl_cache' );
} );

// CORRECT - Delete all data in uninstall.php

Issue #16: Using Deprecated Functions

Error: Plugin breaks on WordPress updates Source: WordPress Deprecated Functions List Why It Happens: Using functions removed in newer WordPress versions Prevention: Enable WP_DEBUG during development

php

// In wp-config.php (development only)
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );

Issue #17: Text Domain Mismatch

Error: Translations don't load Source: WordPress Internationalization Why It Happens: Text domain doesn't match plugin slug Prevention: Use exact plugin slug everywhere

php

// Plugin header
// Text Domain: my-plugin

// In code - MUST MATCH EXACTLY
__( 'Text', 'my-plugin' );
_e( 'Text', 'my-plugin' );

Issue #18: Missing Plugin Dependencies

Error: Fatal error when required plugin is inactive Source: WordPress Plugin Dependencies Why It Happens: No check for required plugins Prevention: Check for dependencies on plugins_loaded

php

add_action( 'plugins_loaded', function() {
    if ( ! class_exists( 'WooCommerce' ) ) {
        add_action( 'admin_notices', function() {
            echo '<div class="error"><p>My Plugin requires WooCommerce.</p></div>';
        } );
        return;
    }
    // Initialize plugin
} );

Issue #19: Autosave Triggering Meta Save

Error: Meta saved multiple times, performance issues Source: WordPress Post Meta Why It Happens: No autosave check in save_post hook Prevention: Check for DOING_AUTOSAVE constant

php

add_action( 'save_post', function( $post_id ) {
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return;
    }

    // Safe to save meta
} );

Issue #20: admin-ajax.php Performance

Error: Slow AJAX responses Source: https://deliciousbrains.com/comparing-wordpress-rest-api-performance-admin-ajax-php/ Why It Happens: admin-ajax.php loads entire WordPress core Prevention: Use REST API for new projects (10x faster)

php

// OLD: admin-ajax.php (still works but slower)
add_action( 'wp_ajax_mypl_action', 'mypl_ajax_handler' );

// NEW: REST API (10x faster, recommended)
add_action( 'rest_api_init', function() {
    register_rest_route( 'myplugin/v1', '/endpoint', array(
        'methods'             => 'POST',
        'callback'            => 'mypl_rest_handler',
        'permission_callback' => function() {
            return current_user_can( 'edit_posts' );
        },
    ) );
} );

Issue #21: Missing show_in_rest for Block Editor

Error: Custom post types show classic editor instead of Gutenberg block editor Source: WordPress VIP Documentation, GitHub Issue #7595 Why It Happens: Forgot to set

show_in_rest => true

when registering custom post type Prevention: Always include show_in_rest for CPTs that need block editor

php

// ❌ WRONG - Block editor won't work
register_post_type( 'book', array(
    'public' => true,
    'supports' => array('editor'),
    // Missing show_in_rest!
) );

// ✅ CORRECT
register_post_type( 'book', array(
    'public' => true,
    'show_in_rest' => true,  // Required for block editor
    'supports' => array('editor'),
) );

Critical Rule: Only post types registered with

'show_in_rest' => true

are compatible with the block editor. The block editor is dependent on the WordPress REST API. For post types that are incompatible with the block editor—or have

show_in_rest => false

—the classic editor will load instead.

Issue #22: wpdb::prepare() Table Name Escaping

Error: SQL syntax error from quoted table names, or hardcoded prefix breaks on different installations Source: WordPress Coding Standards Issue #2442 Why It Happens: Using table names as placeholders adds quotes around the table name Prevention: Table names must NOT be in prepare() placeholders

php

// ❌ WRONG - Adds quotes around table name
$table = $wpdb->prefix . 'my_table';
$wpdb->get_results( $wpdb->prepare(
    "SELECT * FROM %s WHERE id = %d",
    $table, $id
) );
// Result: SELECT * FROM 'wp_my_table' WHERE id = 1
// FAILS - table name is quoted

// ❌ WRONG - Hardcoded prefix
$wpdb->get_results( $wpdb->prepare(
    "SELECT * FROM wp_my_table WHERE id = %d",
    $id
) );
// FAILS if user changed table prefix

// ✅ CORRECT - Table name NOT in prepare()
$table = $wpdb->prefix . 'my_table';
$wpdb->get_results( $wpdb->prepare(
    "SELECT * FROM {$table} WHERE id = %d",
    $id
) );

// ✅ CORRECT - Using wpdb->prefix for built-in tables
$wpdb->get_results( $wpdb->prepare(
    "SELECT * FROM {$wpdb->posts} WHERE ID = %d",
    $id
) );

Issue #23: Nonce Verification Edge Cases

Error: Confusing user experience from nonce failures, or false sense of security Source: MalCare: wp_verify_nonce(), Pressidium: Understanding Nonces Why It Happens: Misunderstanding nonce behavior and limitations Prevention: Understand nonce edge cases and always combine with capability checks

Edge Cases:

Time-Based Return Values:

php

$result = wp_verify_nonce( $nonce, 'action' );
// Returns 1: Valid, generated 0-12 hours ago
// Returns 2: Valid, generated 12-24 hours ago
// Returns false: Invalid or expired

Nonce Reusability: WordPress doesn't track if a nonce has been used. They can be used multiple times within the 12-24 hour window.
Session Invalidation: A nonce is only valid when tied to a valid session. If a user logs out, all their nonces become invalid, causing confusing UX if they had a form open.
Caching Problems: Cache issues can cause mismatches when caching plugins serve an older nonce.
NOT a Substitute for Authorization:

php

// ❌ INSUFFICIENT - Only checks origin, not permission
if ( wp_verify_nonce( $_POST['nonce'], 'delete_user' ) ) {
    delete_user( $_POST['user_id'] );
}

// ✅ CORRECT - Combine with capability check
if ( wp_verify_nonce( $_POST['nonce'], 'delete_user' ) &&
     current_user_can( 'delete_users' ) ) {
    delete_user( absint( $_POST['user_id'] ) );
}

Key Principle (2025): Nonces should never be relied on for authentication or authorization. Always assume nonces can be compromised. Protect your functions using current_user_can().

Issue #24: Hook Priority and Argument Count

Error: Hook callback doesn't receive expected arguments, or runs in wrong order Source: Kinsta: WordPress Hooks Bootcamp Why It Happens: Default is only 1 argument, priority defaults to 10 Prevention: Specify argument count and priority explicitly when needed

php

// ❌ WRONG - Only receives $post_id
add_action( 'save_post', 'my_save_function' );
function my_save_function( $post_id, $post, $update ) {
    // $post and $update are NULL!
}

// ✅ CORRECT - Specify argument count
add_action( 'save_post', 'my_save_function', 10, 3 );
function my_save_function( $post_id, $post, $update ) {
    // Now all 3 arguments are available
}

// Priority matters (lower number = runs earlier)
add_action( 'init', 'first_function', 5 );   // Runs first
add_action( 'init', 'second_function', 10 );  // Default priority
add_action( 'init', 'third_function', 15 );   // Runs last

Best Practices:

Always prefix custom hook names to avoid collisions:

do_action( 'mypl_data_processed' )

not

do_action( 'data_processed' )

Filters must RETURN modified data, not echo it
Hook placement affects backwards compatibility - choose carefully

Issue #25: Custom Post Type URL Conflicts

Error: Individual CPT posts return 404 errors despite permalinks flushed Source: Permalink Manager Pro: URL Conflicts Why It Happens: CPT slug matches a page slug, creating URL conflict Prevention: Use different slug for CPT or rename the page

php

// ❌ CONFLICT - Page and CPT use same slug
// Page URL: example.com/portfolio/
register_post_type( 'portfolio', array(
    'rewrite' => array( 'slug' => 'portfolio' ),
) );
// Individual posts 404: example.com/portfolio/my-project/

// ✅ SOLUTION 1 - Use different slug for CPT
register_post_type( 'portfolio', array(
    'rewrite' => array( 'slug' => 'projects' ),
) );
// Posts: example.com/projects/my-project/
// Page: example.com/portfolio/

// ✅ SOLUTION 2 - Use hierarchical slug
register_post_type( 'portfolio', array(
    'rewrite' => array( 'slug' => 'work/portfolio' ),
) );
// Posts: example.com/work/portfolio/my-project/

// ✅ SOLUTION 3 - Rename the page slug
// Change page from /portfolio/ to /our-portfolio/

Issue #26: WordPress 6.8 bcrypt Password Hashing Migration

Error: Custom password hash handling breaks after WordPress 6.8 upgrade Source: WordPress Core Make, GitHub Issue #21022 Why It Happens: WordPress 6.8+ switched from phpass to bcrypt password hashing Prevention: Use WordPress password functions, don't handle hashes directly

What Changed (WordPress 6.8, April 2025):

Default password hashing algorithm changed from phpass to bcrypt
New hash prefix:
```
$wp$2y$
```
(SHA-384 pre-hashed bcrypt)
Existing passwords automatically rehashed on next login
Popular bcrypt plugins (roots/wp-password-bcrypt) now redundant

php

// ✅ SAFE - These functions continue to work without changes
wp_hash_password( $password );
wp_check_password( $password, $hash );

// ⚠️ NEEDS UPDATE - Direct phpass hash handling
if ( strpos( $hash, '$P$' ) === 0 ) {
    // Custom phpass logic - needs update for bcrypt
}

// ✅ NEW - Detect hash type
if ( strpos( $hash, '$wp$2y$' ) === 0 ) {
    // bcrypt hash (WordPress 6.8+)
} elseif ( strpos( $hash, '$P$' ) === 0 ) {
    // phpass hash (WordPress <6.8)
}

Action Required:

Review plugins that directly handle password hashes
Remove bcrypt plugins when upgrading to 6.8+
No action needed for standard wp_hash_password/wp_check_password usage

Issue #27: WordPress 6.9 WP_Dependencies Deprecation

Error: "Deprecated: Function WP_Dependencies->add_data() was called with an argument that is deprecated" Source: WordPress 6.9 Documentation, WordPress Support Forum Why It Happens: WordPress 6.9 (Dec 2, 2025) deprecated WP_Dependencies object methods Prevention: Test plugins with WP_DEBUG enabled on WordPress 6.9, replace deprecated methods

Affected Plugins (confirmed):

WooCommerce (fixed in 10.4.2)
Yoast SEO (fixed in 26.6)
Elementor (requires 3.24+)

Breaking Changes: WordPress 6.9 removed or modified several deprecated functions that older themes and plugins relied on, breaking custom menu walkers, classic widgets, media modals, and customizer features.

Action Required:

Test plugins with WP_DEBUG enabled on WordPress 6.9
Replace deprecated WP_Dependencies methods
Check for deprecation notices in debug.log
While top 1,000 plugins patched within hours, unmaintained plugins often lag behind

Issue #28: Translation Loading Changes in WordPress 6.7

Error: Translations don't load or debug notices appear Source: WooCommerce Developer Blog, WordPress 6.7 Field Guide Why It Happens: WordPress 6.7+ changed when/how translations load Prevention: Load translations after 'init' priority 10, ensure text domain matches plugin slug

php

// ❌ WRONG - Loading too early
add_action( 'init', 'load_plugin_textdomain' );

// ✅ CORRECT - Load after 'init' priority 10
add_action( 'init', 'load_plugin_textdomain', 11 );

// Ensure text domain matches plugin slug EXACTLY
// Plugin header: Text Domain: my-plugin
__( 'Text', 'my-plugin' );  // Must match exactly

Action Required:

Review when load_plugin_textdomain() is called
Ensure text domain matches plugin slug exactly
Test with WP_DEBUG enabled

Issue #29: wpdb::prepare() Missing Placeholders Error

Error: "The query argument of wpdb::prepare() must have a placeholder" Source: WordPress $wpdb Documentation, SitePoint: Working with Databases Why It Happens: Using prepare() without any placeholders Prevention: Don't use prepare() if no dynamic data

php

// ❌ WRONG
$wpdb->prepare( "SELECT * FROM {$wpdb->posts}" );
// Error: The query argument of wpdb::prepare() must have a placeholder

// ✅ CORRECT - Don't use prepare() if no dynamic data
$wpdb->get_results( "SELECT * FROM {$wpdb->posts}" );

// ✅ CORRECT - Use prepare() for dynamic data
$wpdb->get_results( $wpdb->prepare(
    "SELECT * FROM {$wpdb->posts} WHERE ID = %d",
    $post_id
) );

Additional wpdb::prepare() Mistakes:

Percentage Sign Handling:

php

// ❌ WRONG
$wpdb->prepare( "SELECT * FROM {$wpdb->posts} WHERE post_title LIKE '%test%'" );

// ✅ CORRECT
$search = '%' . $wpdb->esc_like( $term ) . '%';
$wpdb->get_results( $wpdb->prepare(
    "SELECT * FROM {$wpdb->posts} WHERE post_title LIKE %s",
    $search
) );

Mixing Argument Formats:

php

// ❌ WRONG - Can't mix individual args and array
$wpdb->prepare( "... WHERE id = %d AND name = %s", $id, array( $name ) );

// ✅ CORRECT - Pick one format
$wpdb->prepare( "... WHERE id = %d AND name = %s", $id, $name );
// OR
$wpdb->prepare( "... WHERE id = %d AND name = %s", array( $id, $name ) );

Plugin Architecture Patterns

Simple (Functions Only)

Small plugins (<5 functions):

php

function mypl_init() { /* code */ }
add_action( 'init', 'mypl_init' );

OOP (Singleton)

Medium plugins:

php

class MyPL_Plugin {
    private static $instance = null;
    public static function get_instance() {
        if ( null === self::$instance ) self::$instance = new self();
        return self::$instance;
    }
    private function __construct() {
        add_action( 'init', array( $this, 'init' ) );
    }
}
MyPL_Plugin::get_instance();

PSR-4 (Modern, Recommended 2025+)

Large/team plugins:

my-plugin/
├── my-plugin.php
├── composer.json → "psr-4": { "MyPlugin\\": "src/" }
└── src/Admin.php

// my-plugin.php
require_once __DIR__ . '/vendor/autoload.php';
use MyPlugin\Admin;
new Admin();

Common Patterns

Custom Post Types (CRITICAL: Flush rewrite rules on activation, show_in_rest for block editor):

php

// show_in_rest => true REQUIRED for Gutenberg block editor
register_post_type( 'book', array(
    'public' => true,
    'show_in_rest' => true,  // Without this, block editor won't work!
    'supports' => array( 'editor', 'title' ),
) );
register_activation_hook( __FILE__, function() {
    mypl_register_cpt();
    flush_rewrite_rules();  // NEVER call on every page load
} );

Custom Taxonomies:

php

register_taxonomy( 'genre', 'book', array( 'hierarchical' => true, 'show_in_rest' => true ) );

Meta Boxes:

php

add_meta_box( 'book_details', 'Book Details', 'mypl_meta_box_html', 'book' );
// Save: Check nonce, DOING_AUTOSAVE, current_user_can('edit_post')
update_post_meta( $post_id, '_book_isbn', sanitize_text_field( $_POST['book_isbn'] ) );

Settings API:

php

register_setting( 'mypl_options', 'mypl_api_key', array( 'sanitize_callback' => 'sanitize_text_field' ) );
add_settings_section( 'mypl_section', 'API Settings', 'callback', 'my-plugin' );
add_settings_field( 'mypl_api_key', 'API Key', 'field_callback', 'my-plugin', 'mypl_section' );

REST API (10x faster than admin-ajax.php):

php

register_rest_route( 'myplugin/v1', '/data', array(
    'methods'             => 'POST',
    'callback'            => 'mypl_rest_callback',
    'permission_callback' => fn() => current_user_can( 'edit_posts' ),
) );

AJAX (Legacy, use REST API for new projects):

php

add_action( 'wp_ajax_mypl_action', 'mypl_ajax_handler' );
check_ajax_referer( 'mypl-ajax-nonce', 'nonce' );
wp_send_json_success( array( 'message' => 'Success' ) );

Custom Tables:

php

global $wpdb;
$sql = "CREATE TABLE {$wpdb->prefix}mypl_data (id bigint AUTO_INCREMENT PRIMARY KEY, ...)";
require_once ABSPATH . 'wp-admin/includes/upgrade.php';
dbDelta( $sql );

Transients (Caching):

php

$data = get_transient( 'mypl_data' );
if ( false === $data ) {
    $data = expensive_operation();
    set_transient( 'mypl_data', $data, 12 * HOUR_IN_SECONDS );
}

Bundled Resources

Templates:

plugin-simple/

plugin-oop/

plugin-psr4/

examples/meta-box.php

examples/settings-page.php

examples/custom-post-type.php

examples/rest-endpoint.php

examples/ajax-handler.php

Scripts:

scaffold-plugin.sh

check-security.sh

validate-headers.sh

References:

security-checklist.md

hooks-reference.md

sanitization-guide.md

wpdb-patterns.md

common-errors.md

Advanced Topics

i18n (Internationalization):

php

load_plugin_textdomain( 'my-plugin', false, dirname( plugin_basename( __FILE__ ) ) . '/languages' );
__( 'Text', 'my-plugin' );  // Return translated
_e( 'Text', 'my-plugin' );  // Echo translated
esc_html__( 'Text', 'my-plugin' );  // Translate + escape

WP-CLI:

php

if ( defined( 'WP_CLI' ) && WP_CLI ) {
    WP_CLI::add_command( 'mypl', 'MyPL_CLI_Command' );
}

Cron Events:

php

register_activation_hook( __FILE__, fn() => wp_schedule_event( time(), 'daily', 'mypl_daily_task' ) );
register_deactivation_hook( __FILE__, fn() => wp_clear_scheduled_hook( 'mypl_daily_task' ) );
add_action( 'mypl_daily_task', 'mypl_do_daily_task' );

Plugin Dependencies:

php

if ( ! class_exists( 'WooCommerce' ) ) {
    deactivate_plugins( plugin_basename( __FILE__ ) );
    add_action( 'admin_notices', fn() => echo '<div class="error"><p>Requires WooCommerce</p></div>' );
}

Distribution & Auto-Updates

GitHub Auto-Updates (Plugin Update Checker by YahnisElsts):

php

// 1. Install: git submodule add https://github.com/YahnisElsts/plugin-update-checker.git
// 2. Add to main plugin file
require plugin_dir_path( __FILE__ ) . 'plugin-update-checker/plugin-update-checker.php';
use YahnisElsts\PluginUpdateChecker\v5\PucFactory;

$updateChecker = PucFactory::buildUpdateChecker(
    'https://github.com/yourusername/your-plugin/',
    __FILE__,
    'your-plugin-slug'
);
$updateChecker->getVcsApi()->enableReleaseAssets();  // Use GitHub Releases

// Private repos: Define token in wp-config.php
if ( defined( 'YOUR_PLUGIN_GITHUB_TOKEN' ) ) {
    $updateChecker->setAuthentication( YOUR_PLUGIN_GITHUB_TOKEN );
}

Deployment:

bash

git tag 1.0.1 && git push origin main && git push origin 1.0.1
# Create GitHub Release with ZIP (exclude .git, tests)

Alternatives: Git Updater (no coding), Custom Update Server (full control), Freemius (commercial)

Security: Use HTTPS, never hardcode tokens, validate licenses, rate limit update checks

CRITICAL: ZIP must contain plugin folder:

plugin.zip/my-plugin/my-plugin.php

Resources: See

references/github-auto-updates.md

examples/github-updater.php

Dependencies

Required:

WordPress 5.9+ (recommend 6.7+)
PHP 7.4+ (recommend 8.0+)

Optional:

Composer 2.0+ - For PSR-4 autoloading
WP-CLI 2.0+ - For command-line plugin management
Query Monitor - For debugging and performance analysis

Official Documentation

WordPress Plugin Handbook: https://developer.wordpress.org/plugins/
WordPress Coding Standards: https://developer.wordpress.org/coding-standards/
WordPress REST API: https://developer.wordpress.org/rest-api/
WordPress Database Class ($wpdb): https://developer.wordpress.org/reference/classes/wpdb/
WordPress Security: https://developer.wordpress.org/apis/security/
Settings API: https://developer.wordpress.org/plugins/settings/settings-api/
Custom Post Types: https://developer.wordpress.org/plugins/post-types/
Transients API: https://developer.wordpress.org/apis/transients/
Context7 Library ID: /websites/developer_wordpress

Troubleshooting

Fatal Error: Enable WP_DEBUG, check wp-content/debug.log, verify prefixed names, check dependencies

404 on CPT: Flush rewrite rules via Settings → Permalinks → Save

Nonce Fails: Check nonce name/action match, verify not expired (24h default)

AJAX Returns 0/-1: Verify action name matches

wp_ajax_{action}

, check nonce sent/verified

HTML Stripped: Use

wp_kses_post()

not

sanitize_text_field()

for safe HTML

Query Fails: Use

$wpdb->prepare()

, check

$wpdb->prefix

, verify syntax

Complete Setup Checklist

Use this checklist to verify your plugin:

Plugin header complete with all fields
ABSPATH check at top of every PHP file
All functions/classes use unique prefix
All forms have nonce verification
All user input is sanitized
All output is escaped
All database queries use $wpdb->prepare()
Capability checks (not just is_admin())
Custom post types flush rewrite rules on activation
Deactivation hook only clears temporary data
uninstall.php handles permanent cleanup
Text domain matches plugin slug
Scripts/styles only load where needed
WP_DEBUG enabled during development
Tested with Query Monitor for performance
No deprecated function warnings
Works with latest WordPress version

Questions? Issues?

Check
```
references/common-errors.md
```
for extended troubleshooting
Verify all steps in the security foundation
Check official docs: https://developer.wordpress.org/plugins/
Enable WP_DEBUG and check debug.log
Use Query Monitor plugin to debug hooks and queries

Last verified: 2026-01-21 | Skill version: 2.0.0 | Changes: Added 9 new issues from WordPress 6.7-6.9 research (bcrypt migration, WP_Dependencies deprecation, translation loading, REST API CVEs 2025-2026, wpdb::prepare() edge cases, nonce limitations, hook gotchas, CPT URL conflicts). Updated from 20 to 29 documented errors prevented. Error count: 20 → 29. Version: 1.x → 2.0.0 (major version due to significant WordPress version-specific content additions).

wordpress-plugin-core

NPX Install

Tags

SKILL.md Content

WordPress Plugin Development (Core)

Quick Start

Security Foundation (Detailed)

Unique Prefix (4-5 chars minimum)

Capabilities Check (Not is_admin())

Security Trinity (Input → Processing → Output)

Nonces (CSRF Protection)

Prepared Statements

Critical Rules

Always Do

Never Do

Known Issues Prevention

Issue #1: SQL Injection

Issue #2: XSS (Cross-Site Scripting)

Issue #3: CSRF (Cross-Site Request Forgery)

Issue #4: Missing Capability Checks

Issue #5: Direct File Access

Issue #6: Prefix Collision

Issue #7: Rewrite Rules Not Flushed (and Performance)

Issue #8: Transients Not Cleaned

Issue #9: Scripts Loaded Everywhere

Issue #10: Missing Sanitization on Save

Issue #11: Incorrect LIKE Queries

Issue #12: Using extract()

Issue #13: Missing Permission Callback in REST API

Issue #14: Uninstall Hook Registered Repeatedly

Issue #15: Data Deleted on Deactivation

Issue #16: Using Deprecated Functions

Issue #17: Text Domain Mismatch

Issue #18: Missing Plugin Dependencies

Issue #19: Autosave Triggering Meta Save

Issue #20: admin-ajax.php Performance

Issue #21: Missing show_in_rest for Block Editor

Issue #22: wpdb::prepare() Table Name Escaping

Issue #23: Nonce Verification Edge Cases

Issue #24: Hook Priority and Argument Count

Issue #25: Custom Post Type URL Conflicts

Issue #26: WordPress 6.8 bcrypt Password Hashing Migration

Issue #27: WordPress 6.9 WP_Dependencies Deprecation

Issue #28: Translation Loading Changes in WordPress 6.7

Issue #29: wpdb::prepare() Missing Placeholders Error

Plugin Architecture Patterns

Simple (Functions Only)

OOP (Singleton)

PSR-4 (Modern, Recommended 2025+)

Common Patterns

Bundled Resources

Advanced Topics

Distribution & Auto-Updates

Dependencies

Official Documentation

Troubleshooting

Complete Setup Checklist