Install the Zend Optimizer by running the command line Zend Optimizer installer.

At the end of the installer, do not restart Apache yet, since all sites will be unavailable until the settings have been modified.

Next, open “/etc/php.d/ioncube-loader.ini” and comment out the line that looks like this:
“zend_extension=/usr/lib/php/modules//php_ioncube_loader_lin_5.1.so”
use the standard pound (#) sign to do this. Copy this line because you’ll need to add it in the php.ini.

Next, open “/etc/php.ini” (which at this point is actually located at “/usr/local/Zend/etc/php.ini”, relocated by the Zend installer) and go down to just before the [Zend] block. Add the copied zend extension to right above the [Zend] block. It should look similar to this:

zend_extension=/usr/lib/php/modules//php_ioncube_loader_lin_5.1.so
[Zend]
zend_extension_manager.optimizer=/usr/local/Zend/lib/Optimizer-3.2.6
zend_extension_manager.optimizer_ts=/usr/local/Zend/lib/Optimizer_TS-3.2.6
zend_optimizer.version=3.2.6
zend_extension=/usr/local/Zend/lib/ZendExtensionManager.so
zend_extension_ts=/usr/local/Zend/lib/ZendExtensionManager_TS.so

After making these modifications, you can restart Apache (typically by using /etc/init.d/httpd on plesk).

You can verify everything is working initially by doing php-v and then try loading one of the web sites hosting on the server.