if ( !is_multisite() ) {

	wp_redirect( site_url( '/wp-login.php?action=register' ) );//将用户重定向到一个预先制定的绝对URI

die(); }
$wp_query->is_404 = false; //$wp_query 是在wp-blog-header.php文件中定义的一个WP_Query实体对象，它提供了当前请求的信息。->这个符号访问类实例的属性、方法；

// is_multisite()函数源码,是wordpress自己定义的一个函数
function is_multisite() {

	if ( defined( 'MULTISITE' ) )

		return MULTISITE;

	if ( defined( 'SUBDOMAIN_INSTALL' ) || defined( 'VHOST' ) || defined( 'SUNRISE' ) )

		return true;

	return false;

}

//函数源码：

function wp_redirect($location, $status = 302) {

	global $is_IIS;

	/**

	 * Filter the redirect location.

	 *

	 * @since 2.1.0

	 *

	 * @param string $location The path to redirect to.

	 * @param int    $status   Status code to use.

	 */

	$location = apply_filters( 'wp_redirect', $location, $status );

	/**

	 * Filter the redirect status code.

	 *

	 * @since 2.3.0

	 *

	 * @param int    $status   Status code to use.

	 * @param string $location The path to redirect to.

	 */

	$status = apply_filters( 'wp_redirect_status', $status, $location );

	if ( ! $location )

		return false;

	$location = wp_sanitize_redirect($location);

	if ( !$is_IIS && php_sapi_name() != 'cgi-fcgi' )

		status_header($status); // This causes problems on IIS and some FastCGI setups

	header("Location: $location", true, $status);

	return true;

}