If you're stuck using PHP 5.4 or earlier, 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 improve performance a little.
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.