From documentation :

>virtual
>virtual -- Perform an Apache sub-request
>Description
>
>int virtual(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.


But if an header is include, then it's displayed in html. virtual() work without header at all.