php 从m个数中取n个数组合，超出内存怎么办？

下面是我网上找到的代码：

function getCombinationToString($arr,$m)
{
    $result = array();
    if ($m ==1)
    {
        return $arr;
    }

    if ($m == count($arr))
    {
        $result[] = implode(',' , $arr);
        return $result;
    }

    $temp_firstelement = $arr[0];
    unset($arr[0]);
    $arr = array_values($arr);
    $temp_list1 = getCombinationToString($arr, ($m-1));

    foreach ($temp_list1 as $s)
    {
        $s = $temp_firstelement.','.$s;
        $result[] = $s;
    }
    unset($temp_list1);

    $temp_list2 = getCombinationToString($arr, $m);
    foreach ($temp_list2 as $s)
    {
        $result[] = $s;
    }
    unset($temp_list2);

    return $result;
}

$arr=['a','b','c','d','e','f'];
getCombinationToString($arr,3);

输出结果：

Array
(

[0] => a,b,c
[1] => a,b,d
[2] => a,b,e
[3] => a,b,f
[4] => a,c,d
[5] => a,c,e
[6] => a,c,f
[7] => a,d,e
[8] => a,d,f
[9] => a,e,f
[10] => b,c,d
[11] => b,c,e
[12] => b,c,f
[13] => b,d,e
[14] => b,d,f
[15] => b,e,f
[16] => c,d,e
[17] => c,d,f
[18] => c,e,f
[19] => d,e,f

)
现在我的$arr有几百个元素，执行后超出内存，请问该怎么办？有没有人能帮忙改进下组合的函数，我这方面很弱，不知道怎么做？

---

那就不要存啊，生成一个输出一个便可以了