When you open your webpage, are you getting the message that “Zend Optimizer not installed“, when you are pretty sure that it is installed in your server ? You are also sure that nothing has changed in the server and you get this message all of a sudden.

This file was encoded by the Zend Encoder / Zend SafeGuard Suite. In order to run it, please install the freely available Zend Optimizer, version xx.xx or later."

You can verify it from the result of # php -v, if zend optimizer is installed, you will get a result something like this :

“with Zend Optimizer vxx.xx, Copyright (c) 1998-2009, by Zend Technologies”

Even with this result, if you are getting an error that shows optimizer is not installed, then there is an issue with your codes. You might look at the particular PHP file and might see enormous codes. More likely the domain is compromised.  You will need to clean up the site / restore the domain from a clean backup.