The Zend Optimizer is a free product that automagically helps your PHP code go faster.
It does so by performing actual code optimisation for you - changing things around (but leaving the "meaning" of the code the same) to increase execution speed. Even though it runs every time your page is executed, it has little to no noticeable overhead, and can drastically improve performance. Note that there are some cases where using the Optimizer will actually slow things down - usually when scripts are short, and/or exit early. These situations are rare, though, and you need not let this stop you from installing the Optimizer.
Zend Performance Suite comes with Zend Optimizer bundled as it caches the optimized PHP, eliminating the extra overhead of code optimisation.
If you don't plan to use scripts encoded with Zend Safeguard Suite, add the following line to your php.ini file to get a further small boost in performance: