Ok, so you are running PHP 5.1x / 5.2x and need to upgrade to PHP 5.3x in order to support the lastest WordPress 3.x incarnations, but you have a problem – you also have Zend Optimizer (module php-zend-optimizer) installed, there is no support for the standard Zend Optimizer in PHP 5.3x (or later).
Never fear, you can resolve the problem like this in Plesk:
# yum remove php-zend-optimizer
Now you can upgrade php (using the atomic repository), as such:
#yum upgrade php
Now you should install the replacement for Zend Optimizer as follows:
yum install php-zend-guard-loader
# service httpd graceful