Okay I created a completely blank file atctest.php and WP Super Cache breaks using the code:
<!--dynamic-cached-content--><?php
virtual ("/full/path/to/atctest.php");
?><!--
virtual ("/full/path/to/atctest.php");
--><!--/dynamic-cached-content-->
viewing the source code with debug lvl 5 I see
<!--dynamic-cached-content-->
<!-- Page not cached by WP Super Cache. No closing HTML tag. Check your theme. -->
<!--
virtual ("/full/path/to/atctest.php");
--><!--/dynamic-cached-content-->
However, WP Super Cache works fine if I change 'virtual' to 'include':
<!--dynamic-cached-content--><?php
include ("/full/path/to/atctest.php");
?><!--
include ("/full/path/to/atctest.php");
--><!--/dynamic-cached-content-->
Is there some issue with the virtual call? I do need to use virtual for 3rd party support.
I'm using PHP to serve cache files.