Companion Plugins – WordPress Plugin Boilerplate

The plugin exposes a small, stable surface so a companion plugin (for example a WPML or WooCommerce integration shipped as its own plugin) can attach to it safely, without depending on plugin load order or autoloading the core plugin’s classes.

Two primitives, both defined in the bootstrap file:

• the demo_plugin_loaded action — the event “core is ready, attach now”
• the DEMO_PLUGIN_VERSION constant — a cheap presence + version check

The load action

demo-plugin.php

add_action(
  'plugins_loaded',
  static function (): void {
    do_action( 'demo_plugin_loaded', Demo_Plugin::PLUGIN_VERSION );
  },
  0
);

It fires on plugins_loaded at priority 0, and passes the current plugin version as its only argument.

The version constant

demo-plugin.php

if ( ! defined( 'DEMO_PLUGIN_VERSION' ) ) {
  define( 'DEMO_PLUGIN_VERSION', Demo_Plugin::PLUGIN_VERSION );
}

DEMO_PLUGIN_VERSION is a global mirror of Demo_Plugin::PLUGIN_VERSION, which stays the single source of truth. It is the public version contract — the convention WordPress extension developers expect (cf. WC_VERSION, ELEMENTOR_VERSION) — so a companion can presence-check and version-gate with a cheap defined() / version_compare() against a stable name, without coupling to the internal Demo_Plugin class, which core is free to rename or restructure.

Writing a companion plugin

acme-wpml-integration.php

// Initialize once the core plugin signals it is fully loaded.
add_action( 'demo_plugin_loaded', 'acme_wpml_boot' );

function acme_wpml_boot( string $version ): void {
  // Version-gate against the core plugin's public API.
  if ( version_compare( $version, '2.0.0', '<' ) ) {
    return;
  }

  // ...attach to the core plugin's extension points here.
}

A companion that initializes after plugins_loaded priority 0 (rare, but e.g. during late init) can detect core synchronously via the constant and the did_action() guard instead of waiting:

acme-wpml-integration.php

if ( did_action( 'demo_plugin_loaded' )
  && version_compare( DEMO_PLUGIN_VERSION, '2.0.0', '>=' ) ) {
  acme_wpml_boot( DEMO_PLUGIN_VERSION );
}