/templates/cassiopeia/html/
├── com_content/              # Components (com_)
│   ├── article/
│   │   └── default.php
│   └── category/
│       ├── blog.php
│       ├── blog_item.php
│       └── default.php
├── mod_login/                # Modules (mod_)
│   ├── default.php
│   └── slim.php
├── plg_content_pagenavigation/ # Plugins (plg_)
│   └── default.php
└── layouts/                  # Reusable JLayouts
    └── joomla/
        └── content/
            ├── intro_image.php
            └── info_block.php

<?php defined('_JEXEC') or die; ?>
<article class="article-container">
    <header class="article-header">
        <h1><?php echo htmlspecialchars($this->item->title); ?></h1>
    </header>

    <section class="article-body">
        <?php if (!empty($this->item->images)): ?>
            <?php echo JLayoutHelper::render('joomla.content.intro_image',
                ['item' => $this->item]); ?>
        <?php endif; ?>

        <?php echo $this->item->text; ?>
    </section>

    <?php if (!empty($this->item->jcfields)): ?>
        <section class="article-custom-fields">
            <?php foreach ($this->item->jcfields as $field): ?>
                <div class="field-<?php echo htmlspecialchars($field->type); ?>">
                    <strong><?php echo htmlspecialchars($field->label); ?></strong>
                    <?php echo $field->rawvalue; ?>
                </div>
            <?php endforeach; ?>
        </section>
    <?php endif; ?>
</article>

<?php defined('_JEXEC') or die;
$item = $this->item; ?>

<article class="blog-item">
    <h2 class="item-title">
        <?php echo JHtml::_('link', JRoute::_($item->link),
            htmlspecialchars($item->title)); ?>
    </h2>

    <?php if (!empty($item->images)): ?>
        <?php echo JLayoutHelper::render('joomla.content.intro_image',
            ['item' => $item]); ?>
    <?php endif; ?>

    <div class="item-content">
        <?php echo $item->introtext; ?>
    </div>

    <a href="<?php echo JRoute::_($item->link); ?>" class="read-more">
        Read more
    </a>
</article>

<?php defined('_JEXEC') or die;
$params = $this->params; ?>

<form action="<?php echo JRoute::_('index.php'); ?>" method="post" class="login-form">
    <div class="form-group">
        <label for="login-username">Username</label>
        <input type="text" name="username" id="login-username"
               class="form-control" required>
    </div>

    <div class="form-group">
        <label for="login-password">Password</label>
        <input type="password" name="password" id="login-password"
               class="form-control" required>
    </div>

    <button type="submit" class="btn btn-primary">Log In</button>

    <input type="hidden" name="option" value="com_users">
    <input type="hidden" name="task" value="user.login">
    <input type="hidden" name="return" value="<?php echo base64_encode(JUri::current()); ?>">
    <?php echo JHtml::_('form.token'); ?>
</form>

// /templates/cassiopeia/html/mod_login/grid.php
// Alternative grid layout for mod_login
<?php defined('_JEXEC') or die; ?>
<div class="login-grid">
    <!-- grid structure -->
</div>

// /templates/cassiopeia/html/layouts/joomla/custom/article-card.php
<?php defined('_JEXEC') or die;
$title = $displayData['title'] ?? '';
$content = $displayData['content'] ?? '';
$image = $displayData['image'] ?? '';
$link = $displayData['link'] ?? '#';
?>

<article class="card">
    <?php if ($image): ?>
        <figure class="card-image">
            <img src="<?php echo htmlspecialchars($image); ?>"
                 alt="<?php echo htmlspecialchars($title); ?>">
        </figure>
    <?php endif; ?>

    <div class="card-body">
        <h3><?php echo htmlspecialchars($title); ?></h3>
        <div class="card-content">
            <?php echo $content; ?>
        </div>
        <a href="<?php echo htmlspecialchars($link); ?>" class="card-link">
            View more
        </a>
    </div>
</article>

<?php echo JLayoutHelper::render('joomla.custom.article-card', [
    'title' => $this->item->title,
    'content' => $this->item->introtext,
    'image' => json_decode($this->item->images)->image_intro ?? '',
    'link' => JRoute::_($this->item->link),
]); ?>

/templates/cassiopeia-child/
├── html/
│   └── com_content/article/default.php  (custom override)
├── css/
│   └── custom.css
└── templateDetails.xml

<?xml version="1.0" encoding="utf-8"?>
<extension type="template" client="site">
    <name>Cassiopeia Child</name>
    <version>1.0.0</version>
    <description>Child template based on Cassiopeia</description>
    <parent>cassiopeia</parent>

    <files>
        <folder>html</folder>
        <folder>css</folder>
        <filename>templateDetails.xml</filename>
    </files>

    <positions>
        <position>header</position>
        <position>sidebar</position>
        <position>footer</position>
    </positions>
</extension>

// /templates/cassiopeia/html/layouts/com_fields/field/render.php
<?php defined('_JEXEC') or die;
$field = $displayData['field'] ?? null;
$value = $displayData['value'] ?? null;

if (!$field || !$value) return;
?>

<div class="field-container" data-field-id="<?php echo (int)$field->id; ?>">
    <label class="field-label">
        <?php echo htmlspecialchars($field->label); ?>
    </label>
    <div class="field-value">
        <?php echo $value; ?>
    </div>
</div>

<?php
/**
 * Override: Custom article
 *
 * Component: com_content
 * Original view: article/tmpl/default.php
 *
 * CHANGES:
 * - Improved semantic structure
 * - Added custom fields
 * - Reordered metadata
 *
 * DEPENDENCIES: Custom field 'author-bio'
 * JOOMLA: 5.0+
 * DATE: 2024-03-06
 */
defined('_JEXEC') or die;

// GOOD: Escape outputs
<?php echo htmlspecialchars($item->title, ENT_QUOTES, 'UTF-8'); ?>
<?php echo JHtml::_('string.truncate', $item->text, 100); ?>

// GOOD: URLs with JRoute
<?php echo JRoute::_('index.php?option=com_content&view=article&id=' . $item->id); ?>

// BAD: Unescaped output
<?php echo $item->title; ?>

# Compare overrides with core files
diff -u /components/com_content/views/article/tmpl/default.php \
         /templates/cassiopeia/html/com_content/article/default.php

# Backup overrides before updating
cp -r templates/cassiopeia/html templates/cassiopeia/html.backup

# Folders: read+execute
chmod 755 /templates/cassiopeia/html/

# Files: read
chmod 644 /templates/cassiopeia/html/com_content/article/default.php

// Articles
$this->item->id
$this->item->title
$this->item->introtext
$this->item->text
$this->item->images (JSON)
$this->item->jcfields (custom fields)

// Parameters
$this->params->get('show_author')
$this->params->get('show_category')

// Modules
$this->module->id
$this->module->title
$this->params

// JLayout
$displayData (array of passed data)