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I think from a learning curve its best to write as much of your own stuff as possible. Once you get to a certain level, its a case of 'Why re-invent the wheel?'. If there are built in functions out there to cut down on your coding time, use them. Thats what they have been written for.
I don`t use it much myself, but PEAR has got all manner of useful functions.
What other key functions should i take a look at?