$nums = [1, 22, 3, 4, 5, 6, 9, 11, 15];
$target = 15;
function twoSum($nums, $target)
{
    $res = [];
    $first = null;
    foreach ($nums as $k => $val) {
        $another = $target - $val;
        if (!empty($res)) {
            if ($first == $another) {
                $res[] = $k;
                return $res;
            }
        } else {
            if (in_array($another, $nums)) {
                $res[] = $k;
                $first = $val;
            }
        }
    }
    return [];
}
$res = twoSum($nums, $target);
var_dump($res);

运行结果：

array(2) {
  [0] =>
  int(3)
  [1] =>
  int(7)
}