public static function indexDiff(array $array1, array $array2)
    {
        $indexes = array();
        $flippedArray2 = array_flip($array2); # swaps indexes and values
       
        foreach ($array1 as $index => $value)
        {
            if (!isset($flippedArray2[$value])) 
            {
                $indexes[] = $index;
            }
        }
        
        return $indexes;
    }