我想从数据库表中获取用户（当前已登录）的电子邮件和uid，然后将其插入另一个表中。我已经试过了，但我得到的是空白的结果，是在uid和电子邮件。

<?php

    session_start();

     if(isset($_POST['button'])){

     $bidamount = $_POST['bidamount'];
     $email = $_SESSION['$u_email'];
     $uid = $_SESSION['$u_uid'];

     //TO ALERT SUBMISSION OF BLANK FIELDS(IT DOESN'T PREVENT SUBMISSION OF BLANK FIELD THOUGH)
     if (!$bidamount){
         echo "can't submit blank fields";
     }

     //TO CONFIRM YOU ARE CONNECTED TO YOUR DATABASE (OPTIONAL)
     $connection = mysqli_connect('localhost', 'root', '', 'tickmill_auctions');
     if ($connection){
         header ("Location: ../Afterlogin.php?action=success");
     }else{
         die("connection failed");
     }
     //TO INSERT username and password from field to jossyusers database
     $query = "INSERT INTO orders(bidamount,email,uid) VALUES('$bidamount','$email','$uid')";
     $result = mysqli_query($connection, $query);
     if(!$result){
         die("OOPPS! query failed".mysqli_error($connection));
      }
    }

    ?>