在用户添加商品到购物车保存到session后，怎么把购物车的信息存储到数据库

Question

<?php
header("Content-type: text/html; charset=utf-8");
  session_start();//启动session
  // session_destroy();
  // die();
  // var_dump($_SESSION);
  // die();
  if(isset($_SESSION['username']) &&!empty($_SESSION['username'])){
   
  }
  else{
    echo '<script language="JavaScript">;
          alert("尚未登录，请先登录！！");</script>';
          echo "<script>location.href='header.php'</script>";   exit;
  }
  
  // $uid = $_SESSION['uid'];
  $goods_id = $_POST['goods_id'];//获取商品id
  // $size = $_POST['size'];
  // $color = $_POST['color'];
  $number = $_POST['num'];//获取商品数量  // $numcol="$size"."$color";  // var_dump($_POST);
  // die();
  
  
      if(empty($_SESSION["gwc"]))  {      //1.购物车是空的，第一次点击添加购物车      $arr[$goods_id] =array(
          'goods_id'  => $goods_id,
          
          'num'  => $number
          
          
          );      $_SESSION["gwc"]=$arr;      // print_r($arr);
      // die();
  }  else  {      //不是第一次点击      //判断购物车中是否存在该商品      $arr = $_SESSION["gwc"]; //先存一下      $chuxian = false;      //查询购物车中是否有该商品,KEY是商品id
      foreach($arr as $k=> $v)
      {
       
          if($k==$goods_id)
          {              $chuxian = true;          }      }  
// 购物车中已有该商品，则只需将它的数量增加
      if($chuxian)
      {       $arr[$goods_id]['num']+=$number;          //3.如果购物车中有该商品
          // if(!isset($arr[$goods_id])){
          //  $arr[$goods_id]=array(
          //    'id'  => $goods_id,
             
          //    'num'  => $number
             
          
          //    );
          // }else{
          //  $arr[$goods_id]['num']+=$number;
          // }
         
          $_SESSION["gwc"] = $arr;
      }
      else
      {          //2.如果购物车中没有该商品
          $arr[$goods_id] = array(
          'goods_id'  => $goods_id,          
          'num'  => $number          
          );          
          $_SESSION["gwc"] = $arr;         
          // print_r($arr);
          //  die();
      }  }  // echo "OK";
  echo '<script language="JavaScript">;
alert("添加成功！");location.href="header.php";;';
         
        exit;
 // var_dump($_SESSION); //    die ( );
  
#  
 ?>