WP Simple Maintenance Mode

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<?php
/**
 * WP Simple Maintenance Mode plugin
 *
 * Shows site in maintenance mode unless on the login page or one of a 
 * selected group of users.
 * 
 * Revamp of http://pastebin.com/tK6YnfyS
 *
 * @package WP_Simple_Maintenance_Mode
 * @author Brian Richards
 * @author Gary Jones
 * @license GPL3
 * @link https://gist.github.com/1653664
 *
 * @wordpress-plugin
 * Plugin Name: WP Simple Maintenance Mode
 * Plugin URI: https://gist.github.com/1653664
 * Description: Shows site in maintenance mode unless on the login page or one of a selected group of users.
 * Version: 1.0.0
 * Author: <a href="http://rzen.net/">Brian Richards</a> and <a href="http://garyjones.co.uk/">Gary Jones</a>
 * License: GPL3
 */

// Try and load translation file for this plugin.
if( ! load_plugin_textdomain( 'wp_simple_maintenance_mode', false, '/wp-content/languages/' ) )
	load_plugin_textdomain( 'wp_simple_maintenance_mode', false, dirname( plugin_basename( __FILE__ ) ) . '/languages/' );

// Default to WP version if the function exists.
if ( ! function_exists( 'is_login_page' ) ) {
	/**
	 * Check if we're on the login or registration page.
	 * 
	 * @since 1.0.0
	 *
	 * @return boolean True if currently on the login or register page, false otherwise.
	 */
	function is_login_page() {
		
		return in_array( $GLOBALS['pagenow'], array( 'wp-login.php', 'wp-register.php' ) );

	}
}

add_action( 'init', 'wpsmm_show_maintenance_message', 1 );
/**
 * Conditionally enter a simple maintenance mode.
 * 
 * If we're on the login page, or one of the selected user IDs, do nothing.
 * 
 * @since 1.0.0
 *
 * @return null Returns early if on login page or current user is excluded.
 */
function wpsmm_show_maintenance_message() {
	
	if ( is_login_page() )
		return;
	
	$excluded_users = array(
		1, // Admin
		5, // Bob
	);
	
	if ( in_array( get_current_user_id(), $excluded_users ) )
		return;

	// Could send a HTTP header here to tell search engines we're only down temporarily.
	wp_die( __( '<h1>Oops</h1><p>This site is currently undergoing maintenance. Sorry for the inconvenience. Please check back soon.</p>', 'wp_simple_maintenance_mode' ) );

}