The following code may be placed in a "zend_optimizer.ini" file inside the "/etc/php5/conf.d" directory. Debian's php is compiled so it always loads all the ini files there, so this way you don't have to modify any templates, nor fear that a panel update may deactivate it
Code:
[Zend]
zend_extension_manager.optimizer=/usr/local/Zend/lib/Optimizer-3.3.3
zend_extension_manager.optimizer_ts=/usr/local/Zend/lib/Optimizer_TS-3.3.3
zend_optimizer.version=3.3.3
zend_extension=/usr/local/Zend/lib/ZendExtensionManager.so
zend_extension_ts=/usr/local/Zend/lib/ZendExtensionManager_TS.so