LeeCode(PHP) 2.add-two-numbers

class Solution {

    /**
     * @param ListNode $l1
     * @param ListNode $l2
     * @return ListNode
     */
    function addTwoNumbers($l1, $l2) {
        $tmp = [];
        $flag = 0;
        while($flag>0 || $l1->val !== null || $l2->val !== null){
            $num1 = ($l1&&$l1->val)?$l1->val:0;
            $num2 = ($l2&&$l2->val)?$l2->val:0;
            $num = $num1+$num2+$flag;
            $flag = 0;
            if($num>9){
                $flag = 1;
            }
            $tmp[] = $num%10;
            $l1 = $l1->next;
            $l2 = $l2->next;
        }
        $tmp = array_reverse($tmp);
        $res = new ListNode($tmp[0]);
        $res->next = null;
        unset($tmp[0]);
        foreach($tmp as $v){
            $p = new ListNode($v);
            $p->next = $res;
            $res = $p;
        }
        return $res;
    }
}