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.
As of PHP 4.0.6, you can use virtual() on PHP files. However, it is typically better to use include() or require() if you need to include another PHP file.
Note: From PHP 4.3.3 on you can use this function with the NSAPI server module in Netscape/iPlanet/SunONE webservers, too.