PHP's empty()
Two quick notes on the use of the empty() function in PHP.
1) It’s actually a language construct, not a function.
2) As per #1, you can’t do things like
if ( empty( array_intersect( $array1, $array2, $array3 ) ) ) { ... }
If you try, PHP will break with no error message, and you’ll waste approximately two hours trying to figure out why the rest of your function is wrong, when it’s not.