Do not install Zend 2.5 or later versions as they will break all phpbb forums and possibly some others as well. Zend says they have it fixed in Zend 2.5.1 but I have tried this on several servers with no sucess. In order to fix this problem do the following.
# cd /Path/To/Zend
# cp -f php-ini-zendoptimzer.bak php.ini
//Restart Apache and Zend has been removed
//Next step is to re-install a earlier version of Zend, I use 2.1.0
# cd /path/Zend/Was/UnTared
//Follow on screen instructions
//Restart Apache even though install does this
If you are still getting errors in your Forums try clearing your browser's Cache and restarting Apache once more. Always check your work in multiple browsers, I noticed errors in FireFox Mozilla before I got any in IE.