I installed Zend Optimizer 3.2.0 on my RHEL3 box. Installation was OK, according to the Zend installer script. However, it is not OK. Because the Zend Optimizer is not running. Googling around brought me to the Zend forum: http://www.zend.com/forums/index.php?t=rview&th=393&goto=2091 So there seems to be a problem with Zend Optimizer on PHP 4.3.2 Here is the result when i run /usr/bin/php -v Failed loading /usr/local/Zend/lib/Optimizer-3.2.0/php-4.3.x/ZendOptimizer.so: /usr/local/Zend/lib/Optimizer-3.2.0/php-4.3.x/ZendOptimizer.so: undefined symbol: zend_open PHP 4.3.2 (cgi), Copyright (c) 1997-2003 The PHP Group Zend Engine v1.3.0, Copyright (c) 1998-2003 Zend Technologies with the ionCube PHP Loader v2.5, Copyright (c) 2002-2004, by ionCube Ltd., and with Zend Extension Manager v1.0.11, Copyright (c) 2003-2006, by Zend Technologies As you can see some symbol from shared library cannot be resolved, I think the reason why mod_php does not load Zend_optimiser is the same. Has anyone experiences with this? How can I safely upgrade mod_php to 4.3.10 version?