1. If a method can be static, declare it static. Speed improvement is by a factor of 4.
2. echo is faster than print.
3. Set the maxvalue for your for-loops before and not in the loop.
4. Unset your variables to free memory, especially large arrays.
5. Avoid magic like __get, __set, __autoload
6. require_once() is expensive
7. Use full paths in includes and requires, less time spent on resolving the OS paths.
8. If you need to find out the time when the script started executing, $_SERVER['REQUEST_TIME'] is preferred to time()
9. See if you can use strncasecmp, strpbrk and stripos instead of regex
10. str_replace is faster than preg_replace, but strtr is faster than str_replace by a factor of 4