Nic Windley on "[Plugin: W3 Total Cache] MFUNC still not working"

From test I've done it may depend if you're using it with Cloudflare or not;

This is the correct syntax when you are not using Cloudflare.

<!-- mfunc THE CODE WITHOUT PHP TAGS -->
<?php THE CODE ?>
<!-- /mfunc -->

Cloudflare will cause the page to fail to render when it hits the php code so you wouold be better off using mclude.