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. Use echo's multiple parameters instead of string concatenation.
4. Set the maxvalue for your for-loops before and not in the loop.
5. Unset your variables to free memory, especially large arrays.
6. Avoid magic like __get, __set, __autoload
7. require_once() is expensive
8. Use full paths in includes and requires, less time spent on resolving the OS paths.
9. If you need to find out the time when the script started executing, $_SERVER[’REQUEST_TIME’] is preferred to time()
10. See if you can use strncasecmp, strpbrk and stripos instead of regex.

Do not use functions call inside the conditional part of a loop, such as for ($x=0; $x < count($array); $x)
The count() function gets called each time.

Pre increment (++$i) is faster than post increment ($i++). You have to know the difference between both tho, else you might end up drinking 5 cards of panadol, because $a = ++$i will not produce the same result with $a = $i++ .

add yours.