Please check is PHP crache on any page or only on the one.
If PHP craches only on one script I think this is a problem in optimizer. Try to disable optimizer (mmcache.optimizer="0") or throught mmcache.php "Disable Optimizer" and "Clear". If now same file will work - this is an optimizer bug.
If you will diside what this is an optimizer bug I will need your sources to fix MMCahce BUG.