I love finding little thing in PHP that i did not know before. They are generally not important, or advanced but it's still very interesting. Here are a few i found recently:
Select a character with a string without using substr etc:
$string = '1234'; echo $string; // 2
Another one i found was "_haltcompiler", it's one of the ones i have no real use for but interesting nonetheless. It basically stops the PHP compiler so you can just output HTML from then on without any more server resources being used.
I shall no doubt find a lot more of these, and i will have no use for any of them more than likely.