On his blog, Phil Thompson lists seven PHP functions that "saved his life" when developing his apps:
From time to time, I've struggled with minor pieces of coding for what seems like an age and then I've discovered PHP has a ready-made function whose express purpose seems to be to fix my exact problem. Today, I name and honour those PHP functions which saved my life, my career and my sanity.
Functions that made the cut include:
He has his own reasons for each - all of them being handy little functions that fill a specific niche in PHP's vast array of abilities.