Description:
------------
i don't know what is happening
but i guess it can't eveluate (0 - 1) + (0 - 2) by some unknown reason when $n == 0

Test script:
---------------
$wop = function($n) use ( &$wop ){
     //if ($n == 0) return 1; //with this line commented, we get a segfault
     return $wop($n - 1) + $wop($n - 2);
};
print_r( array_map($wop, range(0,20)) );