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)

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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
/**
 * 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):

// 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 ) );

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Security Foundation (Detailed)

Unique Prefix (4-5 chars minimum)

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

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

Capabilities Check (Not is_admin())

// ❌ 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)

// 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)

// 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

// ❌ 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 ) );

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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

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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

// VULNERABLE
$wpdb->query( "DELET

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