用session传值，二维数组怎么只剩一维了？

Question

        <?php 
  foreach ($res as  $value) {
  $sql2="select * from dadao_cart where userid='$userid' and goodid='$goodid'";
  $res2=mysqli_fetch_all(mysqli_query($conn,$sql2));
        $_SESSION['cart']=$res2;
  var_dump($_SESSION['cart']);
        }
  ?>

        <?php 
        require 'conn/conn.php';
        var_dump($_SESSION['cart']);
        ?>

第一页我把数据库的查询结果保存在session里，应该是一个二维的数组：array(1) { [0]=> array(4) { [0]=> string(1) "1" [1]=> string(1) "1" [2]=> string(1) "1" [3]=> string(1) "6" } } array(1) { [0]=> array(4) { [0]=> string(2) "27" [1]=> string(1) "2" [2]=> string(1) "1" [3]=> string(1) "4" } } array(1) { [0]=> array(4) { [0]=> string(2) "31" [1]=> string(1) "3" [2]=> string(1) "1" [3]=> string(1) "1" } } array(1) { [0]=> array(4) { [0]=> string(2) "30" [1]=> string(1) "4" [2]=> string(1) "1" [3]=> string(1) "7" } }，但到了第二页却只显示最后一项

Answer 1

 <?php 

foreach ($res as $value) {
$sql2="select * from dadao_cart where userid='$userid' and goodid='$goodid'";
$res2=mysqli_fetch_all(mysqli_query($conn,$sql2));
$cart=json_encode($res2);
$cart11[] =$cart;
}
$_SESSION['cart'] = $cart11[];
?>
<?php
require 'conn/conn.php';
var_dump($_SESSION['cart']);
?>