Does SSI or PHP use less resources to include one file within another file?

For example:

<?php include 'file.html'; ?>
or

<!--#include virtual="file.html" -->

I couldn't find anything in Google and I don't have any experience with benchmarking of the two.

I can't see any reason why one would be faster than the other though. They are reasonably simple functions and I don't think there is any validation etc. that would slow either down. I think it just comes down to preference.

PHP includes can be a little more powerful though as you able to state mandatory pages for include using require(). You can also include pages off other domains which you can't do via SSI.