wordpress-plugin-core

What this skill does

# WordPress Plugin Development (Core)

**Status**: Production Ready
**Last Updated**: 2025-11-27
**Dependencies**: None (WordPress 5.9+, PHP 7.4+)
**Latest Versions**: WordPress 6.7+, PHP 8.0+ recommended

---

## Quick Start (10 Minutes)

### 1. Choose Plugin Structure

Three architecture patterns available (see `references/plugin-architectures.md` for detailed examples):
- **Simple** (functions only) - Small plugins <5 functions
- **OOP** - Medium plugins with related functionality
- **PSR-4** (Namespaced + Composer) - Modern standard (2025), most maintainable

### 2. Create Plugin Header

Every plugin MUST have a header comment in the main file:

```php
<?php
/**
 * Plugin Name:       My Awesome Plugin
 * Description:       Brief description.
 * Version:           1.0.0
 * Requires at least: 5.9
 * Requires PHP:      7.4
 * Text Domain:       my-plugin
 */

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

**CRITICAL**: Plugin Name is required, Text Domain must match plugin slug exactly.

### 3. Security Foundation (5 Essentials)

```php
// 1. Unique Prefix (4-5 chars)
function mypl_init() { /* code */ }
add_action( 'init', 'mypl_init' );

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

// 3. Nonces for Forms
wp_nonce_field( 'mypl_action', 'mypl_nonce' );

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

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

---

## The 5-Step Security Foundation

### Step 1: Use Unique Prefix for Everything

**Rules**: 4-5 chars minimum, apply to functions, classes, constants, options, transients, meta keys. Avoid `wp_`, `__`, `_`.

```php
// GOOD
function mypl_init() {}
class MyPL_Settings {}
add_option( 'mypl_option', 'value' );

// BAD - Will conflict
function init() {}
class Settings {}
```

### Step 2: Check Capabilities, Not Admin Status

```php
// WRONG
if ( is_admin() ) { /* SECURITY HOLE */ }

// CORRECT
if ( current_user_can( 'manage_options' ) ) { /* Secure */ }
```

**Common Capabilities**: `manage_options` (Admin), `edit_posts` (Editor), `publish_posts` (Author)

### Step 3: The Security Trinity

**Input → Processing → Output** (Sanitize → Validate → Escape):

```php
// SANITIZATION (Input)
$name = sanitize_text_field( $_POST['name'] );
$email = sanitize_email( $_POST['email'] );
$url = esc_url_raw( $_POST['url'] );
$html = wp_kses_post( $_POST['content'] );

// VALIDATION (Logic)
if ( ! is_email( $email ) ) wp_die( 'Invalid email' );

// ESCAPING (Output)
echo esc_html( $name );
echo '<a href="' . esc_url( $url ) . '">' . esc_html( $text ) . '</a>';
```

**Rule**: Sanitize INPUT, escape OUTPUT. Never trust user data.

### Step 4: Nonces (CSRF Protection)

One-time tokens proving requests came from your site.

```php
// Form
<form method="post">
    <?php wp_nonce_field( 'mypl_action', 'mypl_nonce' ); ?>
    <input type="text" name="data" />
</form>

// Verify
if ( ! wp_verify_nonce( $_POST['mypl_nonce'], 'mypl_action' ) ) wp_die( 'Security check failed' );

// AJAX
check_ajax_referer( 'mypl-ajax-nonce', 'nonce' );
```

### Step 5: Prepared Statements for Database

**CRITICAL**: Always use `$wpdb->prepare()` for user input.

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

// CORRECT
$wpdb->get_results( $wpdb->prepare( "SELECT * FROM {$wpdb->prefix}table WHERE id = %d", $_GET['id'] ) );
```

**Placeholders**: `%s` (String), `%d` (Integer), `%f` (Float)

**LIKE Queries**: Use `$wpdb->esc_like()` before adding wildcards:
```php
$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** `<?` or `<?=` - 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 **20** 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;
}
```

### Additional Issues (#6-20)

**For comprehensive error coverage beyond the Top 5**, load `references/error-catalog.md` which includes:
- **Functionality**: Prefix collision, rewrite rules not flushed, deprecated functions, text domain mismatch, plugin dependencies, autosave triggers
- **Performan