wordpress-plugin-core

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
/**
 * 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)

// 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_, __, _.

// 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

// 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):

// 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.

// 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.

// 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:

$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

// 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

// 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()

// 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

// 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

// 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