原因找到了:http://www.turckware.ru/en/e_mmc.htm
Since version 2.3.15, Turck MMCache is compatible with Zend Optimizer's loader. Zend Optimizer must be installed after Turck MMCache in php.ini. If you don't use scripts encoded with Zend Encoder then we do not recommend you install Zend Optimizer with Turck MMCache.