You're better off using [http://www.eaccelerator.net]eaccelerator[/url] as it's the fork of mmcache. MMcache hasn't been updated in just about a year, which has seen some pretty sizable php updates. Eaccelerator, on the other hand came out with just a new version, here in the past couple of months.
This may (or may not) solve your problem, but it's worth trying out. You may also need to add /tmp to the safe mode paths so that users can get to it
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