There it is in the PHP manual for
Note: since return() is a language construct and not a function, the parentheses surrounding its arguments are not required. It is common to leave them out, and you actually should do so as PHP has less work to do in this case.
I knew the parentheses were optional, but I’ve been merrily using them all along. And I probably would have continued doing so until I saw the second note attached to the docs:
Note: You should never use parentheses around your return variable when returning by reference, as this will not work. You can only return variables by reference, not the result of a statement. If you use return ($a); then you’re not returning a variable, but the result of the expression ($a) (which is, of course, the value of $a).