wp_ajax_heartbeat()

function wp_ajax_heartbeat() {
	if ( empty( $_POST['_nonce'] ) ) {
		wp_send_json_error();
	}

	$response = $data = array();
	$nonce_state = wp_verify_nonce( $_POST['_nonce'], 'heartbeat-nonce' );

	// 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'] );
	}

	if ( 1 !== $nonce_state ) {
		$response = apply_filters( 'wp_refresh_nonces', $response, $data, $screen_id );

		if ( false === $nonce_state ) {
			// User is logged in but nonces have expired.
			$response['nonces_expired'] = true;
			wp_send_json( $response );
		}
	}

	if ( ! empty( $data ) ) {
		/**
		 * Filters the Heartbeat response received.
		 *
		 * @since WP-3.6.0
		 *
		 * @param array  $response  The Heartbeat response.
		 * @param array  $data      The $_POST data sent.
		 * @param string $screen_id The screen id.
		 */
		$response = apply_filters( 'heartbeat_received', $response, $data, $screen_id );
	}

	/**
	 * Filters the Heartbeat response sent.
	 *
	 * @since WP-3.6.0
	 *
	 * @param array  $response  The Heartbeat response.
	 * @param string $screen_id The screen id.
	 */
	$response = apply_filters( 'heartbeat_send', $response, $screen_id );

	/**
	 * Fires when Heartbeat ticks in logged-in environments.
	 *
	 * Allows the transport to be easily replaced with long-polling.
	 *
	 * @since WP-3.6.0
	 *
	 * @param array  $response  The Heartbeat response.
	 * @param string $screen_id The screen id.
	 */
	do_action( 'heartbeat_tick', $response, $screen_id );

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

	wp_send_json( $response );
}