PHP多个数组的排列组合,该如何解决

function foo($d) {
  $r = array_pop($d);
  while($d) {
    $t = array();
    $s = array_pop($d);
    if(! is_array($s)) $s = array($s);
    foreach($s as $x) {
      foreach($r as $y) $t[] = array_merge(array($x), is_array($y) ? $y : array($y));
    }
    $r = $t;
  }
  return $r;
}
$ar = array(
  array(1,2,3,4),
  array('a','b','c'),
);
print_r(foo($ar));

Array
(
    [0] => Array
        (
            [0] => 1
            [1] => a
        )

    [1] => Array
        (
            [0] => 1
            [1] => b
        )

    [2] => Array
        (
            [0] => 1
            [1] => c
        )

    [3] => Array
        (
            [0] => 2
            [1] => a
        )

    [4] => Array
        (
            [0] => 2
            [1] => b
        )

    [5] => Array
        (
            [0] => 2
            [1] => c
        )

    [6] => Array
        (
            [0] => 3
            [1] => a
        )

    [7] => Array
        (
            [0] => 3
            [1] => b
        )

    [8] => Array
        (
            [0] => 3
            [1] => c
        )

    [9] => Array
        (
            [0] => 4
            [1] => a
        )

    [10] => Array
        (
            [0] => 4
            [1] => b
        )

    [11] => Array
        (
            [0] => 4
            [1] => c
        )

)