I have PHP Version 4.9, what's the best way to install Zend Optimizer on it and what version should I install?
Well there is no php 4.9. The latest php4 version is 4.4.7. and the latest Zend Optimizer (version 3.3.0) supports php versions from 4.2.x up to 4.4.x. For PHP versions below 4.0.4, use the Zend Optimizer 1.2.0.
To install it, just extract the .tar.gz file you've downloaded and run the install script ./install.sh. You'll be asked for the location of php.ini, apachectl and to restart apache. You can read the FAQ
if you need to d/l an older version of Zend Opt.