Ajax handler for the Heartbeat API in the no-privilege context.

Runs when the user is not logged in.

function wp_ajax_nopriv_heartbeat() {
	$response = array();

	// 'screen_id' is the same as $current_screen->id and the JS global 'pagenow'.
	if ( ! empty( $_POST['screen_id'] ) ) {
		$screen_id = sanitize_key( $_POST['screen_id'] );
	} else {
		$screen_id = 'front';
	}

	if ( ! empty( $_POST['data'] ) ) {
		$data = wp_unslash( (array) $_POST['data'] );

		/**
		 * Filters Heartbeat Ajax response in no-privilege environments.
		 *
		 * @since 3.6.0
		 *
		 * @param array  $response  The no-priv Heartbeat response.
		 * @param array  $data      The $_POST data sent.
		 * @param string $screen_id The screen ID.
		 */
		$response = apply_filters( 'heartbeat_nopriv_received', $response, $data, $screen_id );
	}

	/**
	 * Filters Heartbeat Ajax response in no-privilege environments when no data is passed.
	 *
	 * @since 3.6.0
	 *
	 * @param array  $response  The no-priv Heartbeat response.
	 * @param string $screen_id The screen ID.
	 */
	$response = apply_filters( 'heartbeat_nopriv_send', $response, $screen_id );

	/**
	 * Fires when Heartbeat ticks in no-privilege environments.
	 *
	 * Allows the transport to be easily replaced with long-polling.
	 *
	 * @since 3.6.0
	 *
	 * @param array  $response  The no-priv Heartbeat response.
	 * @param string $screen_id The screen ID.
	 */
	do_action( 'heartbeat_nopriv_tick', $response, $screen_id );

	// Send the current time according to the server.
	$response['server_time'] = time();

	wp_send_json( $response );
}