WP_Hook::add_filter( string $hook_name, callable $callback, int $priority, int $accepted_args )

91	public function add_filter( $hook_name, $callback, $priority, $accepted_args ) {92		if ( null === $priority ) {93			$priority = 0;94		}95 96		$idx = _wp_filter_build_unique_id( $hook_name, $callback, $priority );97		if ( null === $idx ) {98			return;99		}100 101		$priority_existed = isset( $this->callbacks[ $priority ] );102 103		$this->callbacks[ $priority ][ $idx ] = array(104			'function'      => $callback,105			'accepted_args' => (int) $accepted_args,106		);107 108		// If we're adding a new priority to the list, put them back in sorted order.109		if ( ! $priority_existed && count( $this->callbacks ) > 1 ) {110			ksort( $this->callbacks, SORT_NUMERIC );111		}112 113		$this->priorities = array_keys( $this->callbacks );114 115		if ( $this->nesting_level > 0 ) {116			$this->resort_active_iterations( $priority, $priority_existed );117		}118	}