Today I bumped in to this article titled "A HOWTO on Optimizing PHP". It is a nice read. Here is a part of it and a link back to the full article.
A HOWTO on Optimizing PHP
PHP is a very fast programming language, but there is more to optimizing PHP than just speed of code execution.
In this chapter, we explain why optimizing PHP involves many factors which are not code related, and why tuning PHP requires an understanding of how PHP performs in relation to all the other subsystems on your server, and then identifying bottlenecks caused by these subsystems and fixing them. We also cover how to tune and optimize your PHP scripts so they run even faster.
Achieving High Performance
When we talk about good performance, we are not talking about how fast your PHP scripts will run. Performance is a set of tradeoffs between scalability and speed. Scripts tuned to use fewer resources might be slower than scripts that perform caching, but more copies of the same script can be run at one time on a web server.
In the example below, A.php is a sprinter that can run fast, and B.php is a marathon runner than can jog forever at the nearly the same speed. For light loads, A.php is substantially faster, but as the web traffic increases, the performance of B.php only drops a little bit while A.php just runs out of steam.
For full article please visit http://phplens.com/lens/php-book/optimizing-debugging-php.php