PHP doesn't expect you to unset all your variables.
Here your $evenBigger
variable has a local scope, unless you assigned it to some external reference somewhere in your code, you don't need to handle it after the execution of your function.
Note that I don't know exactly the inner behavior of PHP and I can't tell when exactly this will be garbage collected. The doc about is seems nice http://www.php.net/manual/en/features.gc.php