I was having the same problem. When I remove the <?php ?>
tags, the code does not execute.
My ‘solution’ is to comment out the line $output = htmlspecialchars('Invalid mfunc tag syntax. The correct format is: <!-- mfunc PHP code --><!-- /mfunc --> or <!-- mfunc -->PHP code<!-- /mfunc -->.');
on line 1292 in PgCache.php.
I’m sure this is not a recommended solution, but it’s the only one that has worked for me – in previous versions of the plugin as well as in the current.