I would guess that 99% of the time where there is a problem with a PHP script running too slow, it's not the PHP - it's the database and the SQL used to interact with it. However, if you're having systemic PHP performance problems, you might want to look into something like Zend Platform, which includes a number of performance features.
"Please give us a simple answer, so that we don't have to think, because if we think, we might find answers that don't fit the way we want the world to be."
~ Terry Pratchett in Nation
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