WordPress Plugin & Theme Development

Consolidated reference for plugin architecture, lifecycle, settings, data storage, WP-CLI integration, static analysis, coding standards, testing, and deployment.

Resource files (detailed code examples):

• resources/settings-api.md — Full Settings_Page class implementation
• resources/wp-cli.md — WP-CLI custom commands and operations reference
• resources/static-analysis-testing.md — PHPStan, PHPCS, and PHPUnit testing
• resources/build-deploy.md — Build, deploy, and release workflows

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1. Plugin Architecture

Main Plugin File

Every plugin starts with a single PHP file containing the plugin header comment. WordPress reads this header to register the plugin in the admin UI.

<?php
/**
 * Plugin Name: My Awesome Plugin
 * Plugin URI:  https://example.com/my-awesome-plugin
 * Description: A short description of what this plugin does.
 * Version:     1.0.0
 * Author:      Your Name
 * Author URI:  https://example.com
 * License:     GPL-2.0-or-later
 * Text Domain: my-awesome-plugin
 * Domain Path: /languages
 * Requires at least: 6.2
 * Requires PHP: 7.4
 */

// Prevent direct access.
defined( 'ABSPATH' ) || exit;

// Define constants.
define( 'MAP_VERSION', '1.0.0' );
define( 'MAP_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );
define( 'MAP_PLUGIN_URL', plugin_dir_url( __FILE__ ) );
define( 'MAP_PLUGIN_FILE', __FILE__ );

// Autoloader (Composer PSR-4).
if ( file_exists( MAP_PLUGIN_DIR . 'vendor/autoload.php' ) ) {
    require_once MAP_PLUGIN_DIR . 'vendor/autoload.php';
}

// Lifecycle hooks — must be registered at top level, not inside other hooks.
register_activation_hook( __FILE__, [ 'MyAwesomePlugin\\Activator', 'activate' ] );
register_deactivation_hook( __FILE__, [ 'MyAwesomePlugin\\Deactivator', 'deactivate' ] );

// Bootstrap the plugin on `plugins_loaded`.
add_action( 'plugins_loaded', function () {
    MyAwesomePlugin\Plugin::instance()->init();
} );

Bootstrap Pattern

• The main file loads, defines constants, requires the autoloader, registers lifecycle hooks, and defers initialization to a plugins_loaded callback.
• Avoid heavy side effects at file load time. Load on hooks.
• Keep admin-only code behind is_admin() checks or admin-specific hooks.

Namespaces and Autoloading (PSR-4)

Configure in composer.json:

{
    "autoload": {
        "psr-4": {
            "MyAwesomePlugin\\": "includes/"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "MyAwesomePlugin\\Tests\\": "tests/"
        }
    }
}

Run composer dump-autoload after changes.

Folder Structure

my-awesome-plugin/
  my-awesome-plugin.php      # Main plugin file with header
  uninstall.php              # Cleanup on uninstall
  composer.json / phpstan.neon / phpcs.xml / .distignore
  includes/                  # Core PHP classes (PSR-4 root)
    Plugin.php / Activator.php / Deactivator.php
    Admin/  Frontend/  CLI/  REST/
  admin/  public/            # View partials
  assets/  templates/  languages/  tests/

Singleton vs Dependency Injection

Use singleton for the root plugin class only. Prefer dependency injection for everything else (testability).

namespace MyAwesomePlugin;

class Plugin {
    private static ?Plugin $instance = null;

    public static function instance(): Plugin {
        if ( null === self::$instance ) {
            self::$instance = new self();
        }
        return self::$instance;
    }

    public function init(): void {
        ( new Admin\Settings_Page() )->register();
        ( new Frontend\Assets() )->register();
        if ( defined( 'WP_CLI' ) && WP_CLI ) {
            CLI\Commands::register();
        }
    }
}

Action / Filter Hook Architecture

• Actions execute code at a specific point (do_action / add_action).
• Filters modify data and return it (apply_filters / add_filter).
• Always specify the priority (default 10) and accepted args count.
• Provide your own hooks so other developers can extend your plugin.

// Registering hooks in your plugin.
add_action( 'init', [ $this, 'register_post_types' ] );
add_filter( 'the_content', [ $this, 'append_cta' ], 20 );

// Providing extensibility.
$output = apply_filters( 'map_formatted_price', $formatted, $raw_price );
do_action( 'map_after_order_processed', $order_id );

---

2. Lifecycle Hooks

Lifecycle hooks must be registered at top-level scope in the main plugin file, not inside other hooks or conditional blocks.

Activation

namespace MyAwesomePlugin;

class Activator {
    public static function activate(): void {
        self::create_tables();

        if ( false === get_option( 'map_settings' ) ) {
            update_option( 'map_settings', [
                'enabled' => true, 'api_key' => '', 'max_results' => 10,
            ], false );
        }

        update_option( 'map_db_version', MAP_VERSION, false );

        // Register CPTs first, then flush so rules exist.
        ( new Plugin() )->register_post_types();
        flush_rewrite_rules();
    }

    private static function create_tables(): void {
        global $wpdb;
        $table = $wpdb->prefix . 'map_logs';
        $sql = "CREATE TABLE {$table} (
            id bigint(20) unsigned NOT NULL AUTO_INCREMENT,
            user_id bigint(20) unsigned NOT NULL DEFAULT 0,
            action varchar(100) NOT NULL DEFAULT '',
            data longtext NOT NULL DEFAULT '',
            created_at datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
            PRIMARY KEY (id),
            KEY user_id (user_id)
        ) {$wpdb->get_charset_collate()};";

        require_once ABSPATH . 'wp-admin/includes/upgrade.php';
        dbDelta( $sql );
    }
}

Deactivation

namespace MyAwesomePlugin;

class Deactivator {
    public static function deactivate(): void {
        // Clear scheduled cron events.
        wp_clear_scheduled_hook( 'map_daily_cleanup' );
        wp_clear_scheduled_hook( 'map_hourly_sync' );

        // Flush rewrite rules to remove custom rewrites.
        flush_rewrite_rules();
    }
}

Uninstall

Create uninstall.php in the plugin root (preferred over register_uninstall_hook):

<?php
// uninstall.php — runs when plugin is deleted via admin UI.
defined( 'WP_UNINSTALL_PLUGIN' ) || exit;

global $wpdb;
delete_option( 'map_settings' );
delete_option( 'map_db_version' );
$wpdb->query( "DELETE FROM {$wpdb->postmeta} WHERE meta_key LIKE 'map\_%'" );
$wpdb->query( "DROP TABLE IF EXISTS {$wpdb->prefix}map_logs" );
$wpdb->query(
    "DELETE FROM {$wpdb->options} WHERE option_name LIKE '_transient_map_%'
     OR option_name LIKE '_transient_timeout_map_%'"
);
$wpdb->query( "DELETE FROM {$wpdb->usermeta} WHERE meta_key LIKE 'map\_%'" );

Key rule: Never run expensive operations on every request. Activation, deactivation, and uninstall hooks exist precisely so you can perform setup and teardown only when needed.

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3. Settings API

Full Settings_Page class implementation: see resources/settings-api.md.

Options API

$settings = get_option( 'map_settings', [] );          // Read.
update_option( 'map_settings', $new_values );           // Write.
delete_option( 'map_settings' );                        // Delete.
update_option( 'map_large_cache', $data, false );       // autoload=false for infrequent options.

---

4. Data Storage

When to Use What

Storage	Use Case	Size Guidance
Options API	Small config, plugin settings	< 1 MB per option
Post meta	Per-post data tied to a specific post	Keyed per post
User meta	Per-user preferences or state	Keyed per user
Custom tables	Structured, queryable, or large datasets	No practical limit
Transients	Cached data with expiration	Temporary, auto-expires

Custom Tables with dbDelta

dbDelta() rules: each field on its own line, two spaces between name and type, PRIMARY KEY with two spaces before, use KEY not INDEX.

function map_create_table(): void {
    global $wpdb;
    $sql = "CREATE TABLE {$wpdb->prefix}map_events (
        id bigint(20) unsigned NOT NULL AUTO_INCREMENT,
        name varchar(255) NOT NULL DEFAULT '',
        status varchar(20) NOT NULL DEFAULT 'draft',
        event_date datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
        PRIMARY KEY  (id),
        KEY status (status)
    ) {$wpdb->get_charset_collate()};";

    require_once ABSPATH . 'wp-admin/includes/upgrade.php';
    dbDelta( $sql );
}

Schema Upgrades

Store a version and compare on plugins_loaded:

add_action( 'plugins_loaded', function () {
    if ( version_compare( get_option( 'map_db_version', '0' ), MAP_VERSION, '<' ) ) {
        map_create_table();  // dbDelta handles ALTER for existing tables.
        update_option( 'map_db_version', MAP_VERSION, false );
    }
} );

Transients

$data = get_transient( 'map_api_results' );
if ( false === $data ) {
    $data = map_fetch_from_api();
    set_transient( 'map_api_results', $data, HOUR_IN_SECONDS );
}

Safe SQL

Always use $wpdb->prepare() for user input. Never concatenate variables into SQL.

$results = $wpdb->get_results( $wpdb->prepare(
    "SELECT * FROM {$wpdb->prefix}map_events WHERE status = %s AND event_date > %s",
    $status, $date
) );

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5. WP-CLI, Static Analysis, Testing, Build & Deploy

Detailed references in resource files:

• WP-CLI (custom commands, operations reference): resources/wp-cli.md
• PHPStan, PHPCS, Testing: resources/static-analysis-testing.md
• Build & Deploy (Composer, JS/CSS, SVN, GitHub releases): resources/build-deploy.md

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6. Security Checklist

• Sanitize on input, escape on output. Never trust $_POST/$_GET directly.
• Use wp_unslash() before sanitizing superglobals. Read explicit keys only.
• Pair nonce checks with current_user_can(). Nonces prevent CSRF, not authz.
• Use $wpdb->prepare() for all SQL with user input.
• Escape output: esc_html(), esc_attr(), esc_url(), wp_kses_post().

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7. WordPress 6.7-6.9 Breaking Changes

WP 6.7 -- Translation Loading

load_plugin_textdomain() is auto-called for plugins with a Text Domain header. If you call it manually from init, it may load before the preferred translation source (language packs). Move manual calls to after_setup_theme or remove them entirely if the header is set.

WP 6.8 -- Bcrypt Password Hashing

WordPress 6.8 migrates from phpass MD5 to bcrypt (password_hash()). Existing hashes upgrade transparently on next login. Impact on plugins:

• wp_check_password() / wp_hash_password() still work -- use these, never hash passwords manually.
• Plugins that store or compare raw $hash strings from the user_pass column will break if they assume $P$ prefix.
• Custom authentication that bypasses wp_check_password() must handle both $P$ (legacy) and $2y$ (bcrypt) prefixes.

WP 6.9 -- Abilities API & WP_Dependencies Deprecation

• WP_Dependencies class is deprecated. Use wp_enqueue_script() / wp_enqueue_style() -- never instantiate WP_Dependencies directly.
• Abilities API (register_ability()) replaces ad-hoc current_user_can() for REST route permissions (see wp-rest-api skill).

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8. Common Mistakes

Mistake	Why It Fails	Fix
Lifecycle hooks inside add_action('init', ...)	Activation/deactivation hooks not detected	Register at top-level scope in main file
flush_rewrite_rules() without registering CPTs first	Rules flushed before custom post types exist	Call CPT registration, then flush
Missing sanitize_callback on register_setting()	Unsanitized data saved to database	Always provide sanitize callback
autoload left as yes for large/infrequent options	Every page load fetches unused data	Pass false as 4th arg to update_option()
Using $wpdb->query() with string concatenation	SQL injection vulnerability	Use $wpdb->prepare()
Nonce check without capability check	CSRF prevented but no authorization	Always pair nonce + current_user_can()
innerHTML = in admin JS	XSS vector	Use textContent or DOM creation methods
No defined('ABSPATH') guard	Direct file access possible	Add defined('ABSPATH') || exit; to every PHP file
Running dbDelta() on every request	Slow table introspection on every page load	Run only on activation or version upgrade
Not checking WP_UNINSTALL_PLUGIN in uninstall.php	File could be loaded outside uninstall context	Check constant before running cleanup
PHPStan baseline grows unchecked	New errors silently added to baseline	Review baseline changes in PRs; never baseline new code
Missing --dry-run on wp search-replace	Irreversible changes to production database	Always dry-run first, then backup, then run
Forgetting --url= in multisite WP-CLI	Command hits wrong site	Always include --url= for site-specific operations