( string filename )
virtual() is an Apache-specific function which
is equivalent to <!--#include virtual...--> in mod_include.
It performs an Apache sub-request. It is useful for including
CGI scripts or .shtml files, or anything else that you would
parse through Apache. Note that for a CGI script, the script
must generate valid CGI headers. At the minimum that means it
must generate a Content-type header.
To run the sub-request, all buffers are terminated and flushed to the
browser, pending headers are sent too.
此函数仅在 PHP 作为 Apache 模块安装时才可使用。