Well, I was pointed to an article recently, http://reinholdweber.com/?p=3 , which lists 40 tips for php performance.  I decided to put some of those to the test.  Then, I started looking at some other of the common ways to do very common functions.  Here's the results of the tests.  All of these tests were done by measuring either 100,000 loops of each function, or 1,000 loops (for the longer running functions).  By all means, don't take my word for it.  Try it your self!