Great list of tips / hacks for optimizing PHP for speed.    Here are a couple that caught my eye:

1. $row[’id’] is 7 times faster than $row[id]
2. Incrementing an undefined local variable is 9-10 times slower than a pre-initialized one.
3. In some instance you can improve the speed of your code by using an isset() trick.
   Ex.
   if (strlen($foo) < 5) { echo “Foo is too short”; }
   vs.
   if (!isset($foo{5})) { echo “Foo is too short”; }

http://webguy.antaramedia.com/blog/?p=22