Some examples:

1. Use sprintf instead of variables contained in double quotes, it’s about 10x faster.
2. Use pre-calculations, set the maximum value for your for-loops before and not in the loop. ie: for ($x=0; $x < count($array); $x), this calls the count() function each time, use $max=count($array) instead before the for-loop starts.
3. “else if” statements are faster than select statements aka case/switch.
4. ++$i is faster than $ i++, so use pre-increment where possible.

… and many, many others.

I’ll definitely be applying all of these optimisations to my code in the future.

For more information: 50+ PHP optimisation tips revisited