using php virtual

Hi, I’m trying to give a wordpress installation the look of an existing website programmed in .shtml with server side includes. As a test I reprogrammed one of the pages as a php page with the SSI pages running as a <?php virtual (“filename”); ?> That works really well and the page looks correct. However when I try to write a wordpress theme incorporating this I get an error when I preview it through the admin panel,

Warning: virtual() [function.virtual]: Unable to include ‘/var/www/styles/Header.shtml’ – request execution failed in /var/www/wordpress/wp-content/themes/ptheme/header.php on line 77

Does wordpress allow a php virtual call? If not what am I doing wrong. I’m not responsible for the whole site so it can’t be completely rewritten in php.