http://reinholdweber.com/?p=3

7 important for me:

1. If a method can be static, declare it static. Speed improvement is by a factor of 4.
2. Use echo’s multiple parameters instead of string concatenation.
3. Set the maxvalue for your for-loops before and not in the loop.
4. require_once() is expensive.
5. strtr is faster than str_replace by a factor of 4.
6. If you have very time consuming functions in your code, consider writing them as C extensions.
7. Profile your code. A profiler shows you, which parts of your code consumes how many time. The Xdebug debugger already contains a profiler. Profiling shows you the bottlenecks in overview.