example-hook.php

/**
 * Add the Livefyre lf_init_script action to our actions to be modified.
 *
 * @param array $actions Actions to be modified.
 * @return array $actions Actions with our action added.
 */
function modify_livefyre_script_order( $actions ) {
	// Move the Livefyre comments output to load after our scripts.
	$actions[] = array(
		'hook'         => 'wp_footer',
		'method'       => 'lf_init_script',
		'old_priority' => 10,
		'new_priority' => 25,
	);

	return $actions;
}
add_filter( 'modify_action_priority', 'modify_livefyre_script_order' );

modify-actions.php

/**
 * Modify the priority of a particular hooked action.
 *
 * This is useful for cases when dealing with an externally-hooked action (plugin)
 * that hooks a mathod in an unmodifiable way. For example, loading a method in a class
 * outside of the global scope as a child of another non-static class in a constructor.
 *
 * For example, LFAPPS_Comments_Display->lf_init_script().
 *
 * @global array $wp_filter All filters and actions registered.
 */
function modify_action_priority() {
	global $wp_filter;

	/**
	 * Filter to modify any number of actions to another priority.
	 *
	 * Feed in an array to modify an action from one priority to another.
	 *
	 * @param array Arguments for modification [ 'hook', 'method', 'old_priority', 'new_priority' ]
	 */
	$actions = apply_filters( 'modify_action_priority', array() );

	// No actions to modify or $wp_filter is empty, return early.
	if ( empty( $wp_filter ) || empty( $actions ) ) {
		return;
	}

	foreach ( $actions as $action ) {
		// We don't have the correct priority, bump to next action.
		if ( ! isset( $wp_filter[ $action['hook'] ][ $action['old_priority'] ] ) ) {
		   continue;
		}

		// Fetch our actions based on hook and priority.
		$priority_hook_actions = $wp_filter[ $action['hook'] ][ $action['old_priority'] ];
		// Loop through each action hooked into this priority.
		foreach ( $priority_hook_actions as $key => $value ) {
			// If our action matches the hashed key, move it to our next priority.
			if ( strpos( $key, $action['method'] ) > -1 ) {
				// Unset the action from the old priority.
				unset( $wp_filter[ $action['hook'] ][ $action['old_priority'] ][ $key ] );
				// Add the action to the new priority.
				$wp_filter[ $action['hook'] ][ $action['new_priority'] ][ $key ] = $value;
				// No more need to keep looking, break out.
				break;
			}
		}
	}
}
add_action( 'wp_loaded', 'modify_action_priority' );

test-modify-actions.php

/**
 * Class The_Sun_Test_Football_Teams_Query_Functions
 *
 * Test helper functions for retrieving data related to Football Teams.
 */
class The_Sun_Modify_Action_Priority extends WP_UnitTestCase {
	/**
	 * Holds our settings for overriding the action.
	 *
	 * @var array
	 */
	public static $override_details = array(
		'hook'         => 'get_header',
		'method'       => 'function_to_modify',
		'old_priority' => 10,
		'new_priority' => 5,
	);

	/**
	 * Set up the proper hooks and filters.
	 */
	public static function wpSetUpBeforeClass() {
		add_action( self::$override_details['hook'], array( 'The_Sun_Modify_Action_Priority', self::$override_details['method'] ), 10 );
		add_filter( 'modify_action_priority', function( $actions ){
			$actions[] = self::$override_details;
			return $actions;
		}, 15, 1 );

	}

	/**
	 * Action to override - doesn't need to do anything.
	 */
	public static function function_to_modify(){}

	/**
	 * Verify that the action priority has appropriately changed.
	 */
	public function test_modify_action_priority() {
		global $wp_filter;

		// Assert that our action is loaded as part of the test suite.
		$this->assertSame( 1, did_action( 'wp_loaded' ) );

		// Assert that our function is hooked into wp_loaded (returns a priority if running correctly).
		$this->assertNotFalse( has_action( 'wp_loaded', 'modify_action_priority' ) );

		// Run our function manually since it's hooked in too early to be run naturally.
		modify_action_priority();

		// Assert that our new key exists before pulling it for checking our action location.
		$this->assertArrayHasKey( self::$override_details['new_priority'], $wp_filter[ self::$override_details['hook'] ] );

		$correct_priority   = $wp_filter[ self::$override_details['hook'] ][ self::$override_details['new_priority'] ];
		$incorrect_priority = $wp_filter[ self::$override_details['hook'] ][ self::$override_details['old_priority'] ];

		// Get the spl_object_hash - needed to find the right action key.
		$hash = substr( key( $correct_priority ), 0, 32 );

		// Assert that the action hook has been added to the correct priority.
		$this->assertArrayHasKey( $hash . self::$override_details['method'], $correct_priority );
		// Assert that the action has been removed from the old priority.
		$this->assertArrayNotHasKey( $hash . self::$override_details['method'], $incorrect_priority );
	}
}