Brandon Savage has a recommendation fro developers out there - sometimes readability is more important than any micro-efficiency you might gain in your scripts.

Much was made last week over the topic of micro optimization in PHP. Most of these argued that micro optimization was a bad idea. [...] There's another reason that micro optimization can be a bad choice: it makes code absolutely impossible to read!

He points out one example for validating the length of a string in two ways - one using isset to tell which characters in a string are set and the other using the tride and true call to strlen. The first, while benchmarked to give you a (very tiny) improvements, is harder to read at first glance than the check to the string length.