多维数组排序有关问题

$arr = array(array(1,0),array(2,0),array(3,0),array(4,0),array(5,1),array(6,1),array(7,5),array(8,2));

print_r(foo($arr));

function foo($ar, $p=0) {
  $r = array();
  foreach($ar as $v) {
    if($v[1] == $p) {
      $r[] = $v;
      $r = array_merge($r, foo($ar, $v[0]));
    }
  }
  return $r;
}