register_activation_hook() │ WP 2.0.0

Suppose we have a function my_plugin_activate() in the main plugin file: wp-content/plugins/myplugin/myplugin.php, then use this code to run this function during plugin activation:

register_activation_hook( __FILE__, 'my_plugin_activate' );

function my_plugin_activate( $network_wide ) {
	// Activation code ...
}

// NOT working example:
add_action( 'plugins_loaded', 'pluginInit' );

function pluginInit() {

	require_once dirname(__FILE__) . '/includes/Activator.php';

	register_activation_hook( __FILE__, 'Activator', 'activate' ) );
}

If the plugin uses a PHP class, the activation code is added as follows:

register_activation_hook( __FILE__, [ My_Plugin::class, 'install' ] );

class My_Plugin {
	public static function install() {
		// Don't use any output here, for example echo ...
	}
}

If the class that contains the activation function is in a separate file, register the activation function as follows:

include_once __DIR__ . '/class-My_Plugin.php';

register_activation_hook( __FILE__, array( 'My_Plugin', 'on_activate_function' ) );

If you are inside __construct(). Important, __FILE__ must "look" at the main plugin file:

class Activate_Plugin {

	public function __construct(){

		register_activation_hook( PLUGIN_MAIN_FILE_PATH, array( $this, 'my_method' ) );
	}

}

After activating the plugin, only two hooks are triggered: activated_plugin and shutdown.

When you need to do something immediately after activating a plugin, you can pin a function to them.

When this solution is not suitable, you can use WP options: save data to an option and then check if the option is there, and do something if the option is there:

// Main plugin file.

...

register_activation_hook( __FILE__, 'my_plugin_activate' );

add_action( 'admin_init', 'load_plugin' );

function my_plugin_activate( $network_wide ) {

  // add an option to do something later, if there is one.
  add_option( 'Activated_Plugin', 'Plugin-Slug' );

  // Here the activation code ...
}

function load_plugin() {

	if ( is_admin() && get_option( 'Activated_Plugin' ) == 'Plugin-Slug' ) {
		// delete the added option so that it is no longer triggered
		// and do what needs to be done...
		delete_option( 'Activated_Plugin' );

		// Do something once, after activating the plugin
		// For example: add_action('init', 'my_init_function' );
	}

}

Another option to do something during the activation of the plugin is to create your event this way:

register_activation_hook( __FILE__, 'my_plugin_activate' );

function my_plugin_activate(){

	// Set your hook, so that it can be hooked from the plugin files themselves
	do_action( 'my_plugin_activate' );
}

A small plugin that demonstrates how the function should be used:

/*
Plugin Name: A Test
Description: A Test
*/

require_once dirname(__FILE__) . '/my_other_file.php';

/* 
This code will not work. The activation hook must be called from the main file.
register_activation_hook ( dirname(__FILE__) . '/my_other_file.php', 'my_other_function');
*/

// This is a working code.
register_activation_hook( __FILE__, 'test_activated' );

/* 
This is the correct way to declare and access global variables.
Global variables must be declared clearly.
Without this, you won't have access to them.
*/
global $some_var;    
$some_var = 'hey';

// Activation function
function test_activated( $network_wide ){

   // here $some_var will not equal hey
   global $some_var;

   // And here $some_var will be equal to hey
   // This function is defined in the 'my_other_file.php' file
   my_other_function();

   /* 
   This option will not work.

   If you want to write the logs to a temporary file, use fopen/fwrite.

   If you want to check if the activation hook works,
   use exit() inside the hook function.
   */
   echo 'test_activated called!';
}

register_activation_hook( __FILE__, __NAMESPACE__ . '\my_activate_callback' );