array_diff From http://stackoverflow.com/questions/2479963/how-does-array-diff-work/6700430#6700430

array_diff.php

function leo_array_diff($a, $b) {     
  $map = $out = array();     
  foreach($a as $val) $map[$val] = 1;     
  foreach($b as $val) unset($map[$val]);     
  return array_keys($map); 
}