Hidden Features of PHP? [closed]

Documentation. The documentation gets my vote. I haven't encountered a more thorough online documentation for a programming language - everything else I have to piece together from various websites and man pages.

Arrays. Judging from the answers to this question I don't think people fully appreciate just how easy and useful Arrays in PHP are. PHP Arrays act as lists, maps, stacks and generic data structures all at the same time. Arrays are implemented in the language core and are used all over the place which results in good CPU cache locality. Perl and Python both use separate language constructs for lists and maps resulting in more copying and potentially confusing transformations.