WordPress/wp-includes/load.php

<?php
/**
 * These functions are needed to load WordPress.
 *
 * @internal This file must be parsable by PHP4.
 *
 * @package WordPress
 */

/**
 * Turn register globals off.
 *
 * @access private
 * @since 2.1.0
 * @return null Will return null if register_globals PHP directive was disabled
 */
function wp_unregister_GLOBALS() {
	if ( !ini_get( 'register_globals' ) )
		return;

	if ( isset( $_REQUEST['GLOBALS'] ) )
		die( /*WP_I18N_GLOBALS_OVERWRITE*/'GLOBALS overwrite attempt detected'/*/WP_I18N_GLOBALS_OVERWRITE*/ );

	// Variables that shouldn't be unset
	$no_unset = array( 'GLOBALS', '_GET', '_POST', '_COOKIE', '_REQUEST', '_SERVER', '_ENV', '_FILES', 'table_prefix' );

	$input = array_merge( $_GET, $_POST, $_COOKIE, $_SERVER, $_ENV, $_FILES, isset( $_SESSION ) && is_array( $_SESSION ) ? $_SESSION : array() );
	foreach ( $input as $k => $v )
		if ( !in_array( $k, $no_unset ) && isset( $GLOBALS[$k] ) ) {
			$GLOBALS[$k] = null;
			unset( $GLOBALS[$k] );
		}
}

/**
 * Fix $_SERVER variables for various setups.
 *
 * @access private
 * @since 3.0.0
 */
function wp_fix_server_vars() {
	global $PHP_SELF;

	$default_server_values = array(
		'SERVER_SOFTWARE' => '',
		'REQUEST_URI' => '',
	);

	$_SERVER = array_merge( $default_server_values, $_SERVER );

	// Fix for IIS when running with PHP ISAPI
	if ( empty( $_SERVER['REQUEST_URI'] ) || ( php_sapi_name() != 'cgi-fcgi' && preg_match( '/^Microsoft-IIS\//', $_SERVER['SERVER_SOFTWARE'] ) ) ) {

		// IIS Mod-Rewrite
		if ( isset( $_SERVER['HTTP_X_ORIGINAL_URL'] ) ) {
			$_SERVER['REQUEST_URI'] = $_SERVER['HTTP_X_ORIGINAL_URL'];
		}
		// IIS Isapi_Rewrite
		else if ( isset( $_SERVER['HTTP_X_REWRITE_URL'] ) ) {
			$_SERVER['REQUEST_URI'] = $_SERVER['HTTP_X_REWRITE_URL'];
		} else {
			// Use ORIG_PATH_INFO if there is no PATH_INFO
			if ( !isset( $_SERVER['PATH_INFO'] ) && isset( $_SERVER['ORIG_PATH_INFO'] ) )
				$_SERVER['PATH_INFO'] = $_SERVER['ORIG_PATH_INFO'];

			// Some IIS + PHP configurations puts the script-name in the path-info (No need to append it twice)
			if ( isset( $_SERVER['PATH_INFO'] ) ) {
				if ( $_SERVER['PATH_INFO'] == $_SERVER['SCRIPT_NAME'] )
					$_SERVER['REQUEST_URI'] = $_SERVER['PATH_INFO'];
				else
					$_SERVER['REQUEST_URI'] = $_SERVER['SCRIPT_NAME'] . $_SERVER['PATH_INFO'];
			}

			// Append the query string if it exists and isn't null
			if ( ! empty( $_SERVER['QUERY_STRING'] ) ) {
				$_SERVER['REQUEST_URI'] .= '?' . $_SERVER['QUERY_STRING'];
			}
		}
	}

	// Fix for PHP as CGI hosts that set SCRIPT_FILENAME to something ending in php.cgi for all requests
	if ( isset( $_SERVER['SCRIPT_FILENAME'] ) && ( strpos( $_SERVER['SCRIPT_FILENAME'], 'php.cgi' ) == strlen( $_SERVER['SCRIPT_FILENAME'] ) - 7 ) )
		$_SERVER['SCRIPT_FILENAME'] = $_SERVER['PATH_TRANSLATED'];

	// Fix for Dreamhost and other PHP as CGI hosts
	if ( strpos( $_SERVER['SCRIPT_NAME'], 'php.cgi' ) !== false )
		unset( $_SERVER['PATH_INFO'] );

	// Fix empty PHP_SELF
	$PHP_SELF = $_SERVER['PHP_SELF'];
	if ( empty( $PHP_SELF ) )
		$_SERVER['PHP_SELF'] = $PHP_SELF = preg_replace( '/(\?.*)?$/', '', $_SERVER["REQUEST_URI"] );
}

/**
 * Check for the required PHP version, and the MySQL extension or a database drop-in.
 *
 * Dies if requirements are not met.
 *
 * This function must be able to work without a complete environment set up. In wp-load.php, for
 * example, WP_CONTENT_DIR is defined and version.php is included before this function is called.
 *
 * @access private
 * @since 3.0.0
 */
function wp_check_php_mysql_versions() {
	// we can probably extend this function to check if wp_die() exists then use translated strings, and then use it in install.php etc.

	global $required_php_version, $wp_version;
	$php_version = phpversion();
	if ( version_compare( $required_php_version, $php_version, '>' ) )
		die( sprintf( /*WP_I18N_OLD_PHP*/'Your server is running PHP version %1$s but WordPress %2$s requires at least %3$s.'/*/WP_I18N_OLD_PHP*/, $php_version, $wp_version, $required_php_version ) );

	if ( !extension_loaded( 'mysql' ) && !file_exists( WP_CONTENT_DIR . '/db.php' ) )
		die( /*WP_I18N_OLD_MYSQL*/'Your PHP installation appears to be missing the MySQL extension which is required by WordPress.'/*/WP_I18N_OLD_MYSQL*/ );
}

/**
 * Don't load all of WordPress when handling a favicon.ico request.
 * Instead, send the headers for a zero-length favicon and bail.
 *
 * @since 3.0.0
 */
function wp_favicon_request() {
	if ( '/favicon.ico' == $_SERVER['REQUEST_URI'] ) {
		header('Content-Type: image/vnd.microsoft.icon');
		header('Content-Length: 0');
		exit;
	}
}

/**
 * Dies with a maintenance message when conditions are met.
 *
 * Checks for a file in the WordPress root directory named ".maintenance".
 * This file will contain the variable $upgrading, set to the time the file
 * was created. If the file was created less than 10 minutes ago, WordPress
 * enters maintenance mode and displays a message.
 *
 * The default message can be replaced by using a drop-in (maintenance.php in
 * the wp-content directory).
 *
 * @access private
 * @since 3.0.0
 */
function wp_maintenance() {
	if ( !file_exists( ABSPATH . '.maintenance' ) || defined( 'WP_INSTALLING' ) )
		return;

	global $upgrading;

	include( ABSPATH . '.maintenance' );
	// If the $upgrading timestamp is older than 10 minutes, don't die.
	if ( ( time() - $upgrading ) >= 600 )
		return;

	if ( file_exists( WP_CONTENT_DIR . '/maintenance.php' ) ) {
		require_once( WP_CONTENT_DIR . '/maintenance.php' );
		die();
	}

	$protocol = $_SERVER["SERVER_PROTOCOL"];
	if ( 'HTTP/1.1' != $protocol && 'HTTP/1.0' != $protocol )
		$protocol = 'HTTP/1.0';
	header( "$protocol 503 Service Unavailable", true, 503 );
	header( 'Content-Type: text/html; charset=utf-8' );
	header( 'Retry-After: 600' );
?>
	<!DOCTYPE html>
	<html xmlns="http://www.w3.org/1999/xhtml">
	<head>
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
		<title><?php echo /*WP_I18N_MAINTENANCE*/'Maintenance'/*/WP_I18N_MAINTENANCE*/; ?></title>

	</head>
	<body>
		<h1><?php echo /*WP_I18N_MAINT_MSG*/'Briefly unavailable for scheduled maintenance. Check back in a minute.'/*/WP_I18N_MAINT_MSG*/; ?></h1>
	</body>
	</html>
<?php
	die();
}

/**
 * PHP 5 standard microtime start capture.
 *
 * @access private
 * @since 0.71
 * @global float $timestart Seconds from when function is called.
 * @return bool Always returns true.
 */
function timer_start() {
	global $timestart;
	$timestart = microtime( true );
	return true;
}

/**
 * Return and/or display the time from the page start to when function is called.
 *
 * You can get the results and print them by doing:
 * <code>
 * $nTimePageTookToExecute = timer_stop();
 * echo $nTimePageTookToExecute;
 * </code>
 *
 * Or instead, you can do:
 * <code>
 * timer_stop(1);
 * </code>
 * which will do what the above does. If you need the result, you can assign it to a variable, but
 * in most cases, you only need to echo it.
 *
 * @since 0.71
 * @global float $timestart Seconds from when timer_start() is called
 * @global float $timeend Seconds from when function is called
 *
 * @param int $display Use '0' or null to not echo anything and 1 to echo the total time
 * @param int $precision The amount of digits from the right of the decimal to display. Default is 3.
 * @return float The "second.microsecond" finished time calculation
 */
function timer_stop( $display = 0, $precision = 3 ) { // if called like timer_stop(1), will echo $timetotal
	global $timestart, $timeend;
	$timeend = microtime( true );
	$timetotal = $timeend - $timestart;
	$r = ( function_exists( 'number_format_i18n' ) ) ? number_format_i18n( $timetotal, $precision ) : number_format( $timetotal, $precision );
	if ( $display )
		echo $r;
	return $r;
}

/**
 * Sets PHP error handling and handles WordPress debug mode.
 *
 * Uses three constants: WP_DEBUG, WP_DEBUG_DISPLAY, and WP_DEBUG_LOG. All three can be
 * defined in wp-config.php. Example: <code> define( 'WP_DEBUG', true ); </code>
 *
 * WP_DEBUG_DISPLAY and WP_DEBUG_LOG perform no function unless WP_DEBUG is true.
 * WP_DEBUG defaults to false.
 *
 * When WP_DEBUG is true, all PHP notices are reported. WordPress will also display
 * notices, including one when a deprecated WordPress function, function argument,
 * or file is used. Deprecated code may be removed from a later version.
 *
 * It is strongly recommended that plugin and theme developers use WP_DEBUG in their
 * development environments.
 *
 * When WP_DEBUG_DISPLAY is true, WordPress will force errors to be displayed.
 * WP_DEBUG_DISPLAY defaults to true. Defining it as null prevents WordPress from
 * changing the global configuration setting. Defining WP_DEBUG_DISPLAY as false
 * will force errors to be hidden.
 *
 * When WP_DEBUG_LOG is true, errors will be logged to wp-content/debug.log.
 * WP_DEBUG_LOG defaults to false.
 *
 * @access private
 * @since 3.0.0
 */
function wp_debug_mode() {
	if ( WP_DEBUG ) {
		// E_DEPRECATED is a core PHP constant in PHP 5.3. Don't define this yourself.
		// The two statements are equivalent, just one is for 5.3+ and for less than 5.3.
//		if ( defined( 'E_DEPRECATED' ) )
//			error_reporting( E_ALL & ~E_DEPRECATED & ~E_STRICT );
//		else
			error_reporting( E_ALL | E_DEPRECATED | E_STRICT );

		if ( WP_DEBUG_DISPLAY )
			ini_set( 'display_errors', 1 );
		elseif ( null !== WP_DEBUG_DISPLAY )
			ini_set( 'display_errors', 0 );

		if ( WP_DEBUG_LOG ) {
			ini_set( 'log_errors', 1 );
			ini_set( 'error_log', WP_CONTENT_DIR . '/debug.log' );
		}
	} else {
		error_reporting( E_CORE_ERROR | E_CORE_WARNING | E_COMPILE_ERROR | E_ERROR | E_WARNING | E_PARSE | E_USER_ERROR | E_USER_WARNING | E_RECOVERABLE_ERROR );
	}
}

/**
 * Sets the location of the language directory.
 *
 * To set directory manually, define <code>WP_LANG_DIR</code> in wp-config.php.
 *
 * If the language directory exists within WP_CONTENT_DIR, that is used.
 * Otherwise if the language directory exists within WPINC, that's used.
 * Finally, if neither of the preceding directories are found,
 * WP_CONTENT_DIR/languages is used.
 *
 * The WP_LANG_DIR constant was introduced in 2.1.0.
 *
 * @access private
 * @since 3.0.0
 */
function wp_set_lang_dir() {
	if ( !defined( 'WP_LANG_DIR' ) ) {
		if ( file_exists( WP_CONTENT_DIR . '/languages' ) && @is_dir( WP_CONTENT_DIR . '/languages' ) || !@is_dir(ABSPATH . WPINC . '/languages') ) {
			define( 'WP_LANG_DIR', WP_CONTENT_DIR . '/languages' ); // no leading slash, no trailing slash, full path, not relative to ABSPATH
			if ( !defined( 'LANGDIR' ) ) {
				// Old static relative path maintained for limited backwards compatibility - won't work in some cases
				define( 'LANGDIR', 'wp-content/languages' );
			}
		} else {
			define( 'WP_LANG_DIR', ABSPATH . WPINC . '/languages' ); // no leading slash, no trailing slash, full path, not relative to ABSPATH
			if ( !defined( 'LANGDIR' ) ) {
				// Old relative path maintained for backwards compatibility
				define( 'LANGDIR', WPINC . '/languages' );
			}
		}
	}
}

/**
 * Load the correct database class file.
 *
 * This function is used to load the database class file either at runtime or by
 * wp-admin/setup-config.php. We must globalize $wpdb to ensure that it is
 * defined globally by the inline code in wp-db.php.
 *
 * @since 2.5.0
 * @global $wpdb WordPress Database Object
 */
function require_wp_db() {
	global $wpdb;

	require_once( ABSPATH . WPINC . '/wp-db.php' );
	if ( file_exists( WP_CONTENT_DIR . '/db.php' ) )
		require_once( WP_CONTENT_DIR . '/db.php' );

	if ( isset( $wpdb ) )
		return;

	$wpdb = new wpdb( DB_USER, DB_PASSWORD, DB_NAME, DB_HOST );
}

/**
 * Sets the database table prefix and the format specifiers for database table columns.
 *
 * Columns not listed here default to %s.
 *
 * @see wpdb::$field_types Since 2.8.0
 * @see wpdb::prepare()
 * @see wpdb::insert()
 * @see wpdb::update()
 * @see wpdb::set_prefix()
 *
 * @access private
 * @since 3.0.0
 */
function wp_set_wpdb_vars() {
	global $wpdb, $table_prefix;
	if ( !empty( $wpdb->error ) )
		dead_db();

	$wpdb->field_types = array( 'post_author' => '%d', 'post_parent' => '%d', 'menu_order' => '%d', 'term_id' => '%d', 'term_group' => '%d', 'term_taxonomy_id' => '%d',
		'parent' => '%d', 'count' => '%d','object_id' => '%d', 'term_order' => '%d', 'ID' => '%d', 'commment_ID' => '%d', 'comment_post_ID' => '%d', 'comment_parent' => '%d',
		'user_id' => '%d', 'link_id' => '%d', 'link_owner' => '%d', 'link_rating' => '%d', 'option_id' => '%d', 'blog_id' => '%d', 'meta_id' => '%d', 'post_id' => '%d',
		'user_status' => '%d', 'umeta_id' => '%d', 'comment_karma' => '%d', 'comment_count' => '%d',
		// multisite:
		'active' => '%d', 'cat_id' => '%d', 'deleted' => '%d', 'lang_id' => '%d', 'mature' => '%d', 'public' => '%d', 'site_id' => '%d', 'spam' => '%d',
	);

	$prefix = $wpdb->set_prefix( $table_prefix );

	if ( is_wp_error( $prefix ) )
		wp_die( /*WP_I18N_BAD_PREFIX*/'<strong>ERROR</strong>: <code>$table_prefix</code> in <code>wp-config.php</code> can only contain numbers, letters, and underscores.'/*/WP_I18N_BAD_PREFIX*/ );
}

/**
 * Starts the WordPress object cache.
 *
 * If an object-cache.php file exists in the wp-content directory,
 * it uses that drop-in as an external object cache.
 *
 * @access private
 * @since 3.0.0
 */
function wp_start_object_cache() {
	global $_wp_using_ext_object_cache;

	$first_init = false;
 	if ( ! function_exists( 'wp_cache_init' ) ) {
		if ( file_exists( WP_CONTENT_DIR . '/object-cache.php' ) ) {
			require_once ( WP_CONTENT_DIR . '/object-cache.php' );
			$_wp_using_ext_object_cache = true;
		} else {
			require_once ( ABSPATH . WPINC . '/cache.php' );
			$_wp_using_ext_object_cache = false;
		}
		$first_init = true;
	} else if ( !$_wp_using_ext_object_cache && file_exists( WP_CONTENT_DIR . '/object-cache.php' ) ) {
		// Sometimes advanced-cache.php can load object-cache.php before it is loaded here.
		// This breaks the function_exists check above and can result in $_wp_using_ext_object_cache
		// being set incorrectly. Double check if an external cache exists.
		$_wp_using_ext_object_cache = true;
	}

	// If cache supports reset, reset instead of init if already initialized.
	// Reset signals to the cache that global IDs have changed and it may need to update keys
	// and cleanup caches.
	if ( !$first_init && function_exists('wp_cache_reset') )
		wp_cache_reset();
	else
		wp_cache_init();

	if ( function_exists( 'wp_cache_add_global_groups' ) ) {
		wp_cache_add_global_groups( array( 'users', 'userlogins', 'usermeta', 'user_meta', 'site-transient', 'site-options', 'site-lookup', 'blog-lookup', 'blog-details', 'rss', 'global-posts' ) );
		wp_cache_add_non_persistent_groups( array( 'comment', 'counts', 'plugins' ) );
	}
}

/**
 * Redirects to the installer if WordPress is not installed.
 *
 * Dies with an error message when multisite is enabled.
 *
 * @access private
 * @since 3.0.0
 */
function wp_not_installed() {
	if ( is_multisite() ) {
		if ( ! is_blog_installed() && ! defined( 'WP_INSTALLING' ) )
			wp_die( __( 'The site you have requested is not installed properly. Please contact the system administrator.' ) );
	} elseif ( ! is_blog_installed() && false === strpos( $_SERVER['PHP_SELF'], 'install.php' ) && !defined( 'WP_INSTALLING' ) ) {

		$link = wp_guess_url() . '/wp-admin/install.php';

		require( ABSPATH . WPINC . '/kses.php' );
		require( ABSPATH . WPINC . '/pluggable.php' );
		require( ABSPATH . WPINC . '/formatting.php' );
		wp_redirect( $link );
		die();
	}
}

/**
 * Returns array of must-use plugin files to be included in global scope.
 *
 * The default directory is wp-content/mu-plugins. To change the default directory
 * manually, define <code>WPMU_PLUGIN_DIR</code> and <code>WPMU_PLUGIN_URL</code>
 * in wp-config.php.
 *
 * @access private
 * @since 3.0.0
 * @return array Files to include
 */
function wp_get_mu_plugins() {
	$mu_plugins = array();
	if ( !is_dir( WPMU_PLUGIN_DIR ) )
		return $mu_plugins;
	if ( ! $dh = opendir( WPMU_PLUGIN_DIR ) )
		return $mu_plugins;
	while ( ( $plugin = readdir( $dh ) ) !== false ) {
		if ( substr( $plugin, -4 ) == '.php' )
			$mu_plugins[] = WPMU_PLUGIN_DIR . '/' . $plugin;
	}
	closedir( $dh );
	sort( $mu_plugins );

	return $mu_plugins;
}

/**
 * Returns array of plugin files to be included in global scope.
 *
 * The default directory is wp-content/plugins. To change the default directory
 * manually, define <code>WP_PLUGIN_DIR</code> and <code>WP_PLUGIN_URL</code>
 * in wp-config.php.
 *
 * @access private
 * @since 3.0.0
 * @return array Files to include
 */
function wp_get_active_and_valid_plugins() {
	$plugins = array();
	$active_plugins = (array) get_option( 'active_plugins', array() );

	// Check for hacks file if the option is enabled
	if ( get_option( 'hack_file' ) && file_exists( ABSPATH . 'my-hacks.php' ) ) {
		_deprecated_file( 'my-hacks.php', '1.5' );
		array_unshift( $plugins, ABSPATH . 'my-hacks.php' );
	}

	if ( empty( $active_plugins ) || defined( 'WP_INSTALLING' ) )
		return $plugins;

	$network_plugins = is_multisite() ? wp_get_active_network_plugins() : false;

	foreach ( $active_plugins as $plugin ) {
		if ( ! validate_file( $plugin ) // $plugin must validate as file
			&& '.php' == substr( $plugin, -4 ) // $plugin must end with '.php'
			&& file_exists( WP_PLUGIN_DIR . '/' . $plugin ) // $plugin must exist
			// not already included as a network plugin
			&& ( ! $network_plugins || ! in_array( WP_PLUGIN_DIR . '/' . $plugin, $network_plugins ) )
			)
		$plugins[] = WP_PLUGIN_DIR . '/' . $plugin;
	}
	return $plugins;
}

/**
 * Sets internal encoding using mb_internal_encoding().
 *
 * In most cases the default internal encoding is latin1, which is of no use,
 * since we want to use the mb_ functions for utf-8 strings.
 *
 * @access private
 * @since 3.0.0
 */
function wp_set_internal_encoding() {
	if ( function_exists( 'mb_internal_encoding' ) ) {
		if ( !@mb_internal_encoding( get_option( 'blog_charset' ) ) )
			mb_internal_encoding( 'UTF-8' );
	}
}

/**
 * Add magic quotes to $_GET, $_POST, $_COOKIE, and $_SERVER.
 *
 * Also forces $_REQUEST to be $_GET + $_POST. If $_SERVER, $_COOKIE,
 * or $_ENV are needed, use those superglobals directly.
 *
 * @access private
 * @since 3.0.0
 */
function wp_magic_quotes() {
	// If already slashed, strip.
	if ( get_magic_quotes_gpc() ) {
		$_GET    = stripslashes_deep( $_GET    );
		$_POST   = stripslashes_deep( $_POST   );
		$_COOKIE = stripslashes_deep( $_COOKIE );
	}

	// Escape with wpdb.
	$_GET    = add_magic_quotes( $_GET    );
	$_POST   = add_magic_quotes( $_POST   );
	$_COOKIE = add_magic_quotes( $_COOKIE );
	$_SERVER = add_magic_quotes( $_SERVER );

	// Force REQUEST to be GET + POST.
	$_REQUEST = array_merge( $_GET, $_POST );
}

/**
 * Runs just before PHP shuts down execution.
 *
 * @access private
 * @since 1.2.0
 */
function shutdown_action_hook() {
	do_action( 'shutdown' );
	wp_cache_close();
}

/**
 * Copy an object.
 *
 * @since 2.7.0
 * @deprecated 3.2
 *
 * @param object $object The object to clone
 * @return object The cloned object
 */

function wp_clone( $object ) {
	// Use parens for clone to accommodate PHP 4. See #17880
	return clone( $object );
}

/**
 * Whether the current request is for a network or blog admin page
 *
 * Does not inform on whether the user is an admin! Use capability checks to
 * tell if the user should be accessing a section or not.
 *
 * @since 1.5.1
 *
 * @return bool True if inside WordPress administration pages.
 */
function is_admin() {
	if ( defined( 'WP_ADMIN' ) )
		return WP_ADMIN;
	return false;
}

/**
 * Whether the current request is for a blog admin screen /wp-admin/
 *
 * Does not inform on whether the user is a blog admin! Use capability checks to
 * tell if the user should be accessing a section or not.
 *
 * @since 3.1.0
 *
 * @return bool True if inside WordPress network administration pages.
 */
function is_blog_admin() {
	if ( defined( 'WP_BLOG_ADMIN' ) )
		return WP_BLOG_ADMIN;
	return false;
}

/**
 * Whether the current request is for a network admin screen /wp-admin/network/
 *
 * Does not inform on whether the user is a network admin! Use capability checks to
 * tell if the user should be accessing a section or not.
 *
 * @since 3.1.0
 *
 * @return bool True if inside WordPress network administration pages.
 */
function is_network_admin() {
	if ( defined( 'WP_NETWORK_ADMIN' ) )
		return WP_NETWORK_ADMIN;
	return false;
}

/**
 * Whether the current request is for a user admin screen /wp-admin/user/
 *
 * Does not inform on whether the user is an admin! Use capability checks to
 * tell if the user should be accessing a section or not.
 *
 * @since 3.1.0
 *
 * @return bool True if inside WordPress user administration pages.
 */
function is_user_admin() {
	if ( defined( 'WP_USER_ADMIN' ) )
		return WP_USER_ADMIN;
	return false;
}

/**
 * Whether Multisite support is enabled
 *
 * @since 3.0.0
 *
 * @return bool True if multisite is enabled, false otherwise.
 */
function is_multisite() {
	if ( defined( 'MULTISITE' ) )
		return MULTISITE;

	if ( defined( 'SUBDOMAIN_INSTALL' ) || defined( 'VHOST' ) || defined( 'SUNRISE' ) )
		return true;

	return false;
}

?>