Buy a book or two. Books are peer-reviewed and edited by experts, online resources usually aren't. However, they can become out of date with each new release of php, but the basics should be the same. Good publishers include Wrox, Apress, and O'Reilly.
I hate to rubbish anything, but be very careful with w3schools, it still contains a few errors.
The php.net manual is an essential reference for almost everything, although some functions are poorly documented, but even those should have some user-based examples of use.
diafol
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