问个PHP数组合并的有关问题~请

$arr1=array(
            "0"=>array(
                'id' => '2',
                'b' => 0,
                'num' => 4

                ),

            "1"=>array(
                'id' => '1',
                'b' => 0,
                'num' => 4 
                )
        );
        $arr2=array(
            "1"=>array(
                'id' => '3',
                'b' => 0,
                'num' => 4

                ),

            "2"=>array(
                'id' => '2',
                'b' => 0,
                'num' => 4 
                )
        );

$arr1=array(
            "0"=>array(
                'id' => '2',
                'b' => 0,
                'num' => 4
 
                ),
 
            "1"=>array(
                'id' => '1',
                'b' => 0,
                'num' => 4 
                )
        );
 $arr2=array(
            "1"=>array(
                'id' => '3',
                'b' => 0,
                'num' => 4
 
                ),
 
            "2"=>array(
                'id' => '2',
                'b' => 0,
                'num' => 4 
                )
        );
foreach($arr1 as $v) $res["$v[id]_$v[b]"] = $v;
foreach($arr2 as $v)
  if(isset($res["$v[id]_$v[b]"])) $res["$v[id]_$v[b]"]['num'] += $v['num'];
  else $res["$v[id]_$v[b]"] = $v;
print_r(array_values($res));

Array
(
    [0] => Array
        (
            [id] => 2
            [b] => 0
            [num] => 8
        )

    [1] => Array
        (
            [id] => 1
            [b] => 0
            [num] => 4
        )

    [2] => Array
        (
            [id] => 3
            [b] => 0
            [num] => 4
        )

)