更改密码而不更改密码

Question

我对此代码有疑问。因此，当用户登录并想要更改其密码时，它们似乎不会更改数据库中的密码。我已设法摆脱错误消息，所以没有错误出现从来没有，当我按下提交数据库中的密码更改为0.当所有三个字段已填写并且格式正确时，消息显示说明密码已更改。但是，其他验证不起作用。在我的数据库中，我有以下字段; 全名， 电子邮件， 密码， 密码2。

<?php
session_start(); 
if (!isset($_SESSION["user_login"])) {
   header("Location: sign_up.php");
} else {
      $username = $_SESSION["user_login"];
}

include ("connect.php");


?>

<?php

//Variables

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

$oldpassword = $_POST['oldpassword'];
$newpassword1 = $_POST['newpassword1'];
$newpassword2 = $_POST['newpassword2'];




    $pass_query  = mysqli_query ($connect, "SELECT * FROM users WHERE email='$username'");
    while ($row = mysqli_fetch_assoc($pass_query)) {

        $existing_pass = $row ['password'];


        //Checking if md5 encrypted password matches
        $md5_oldpassword = md5($oldpassword);
        //check if the old password and the old password entered now match
        if ($md5_oldpassword == $existing_pass){
            //check if the two new passwords match 
            if ($newpassword1 == $newpassword2) {

                $md5_newpassword = md5($newpassword1);
                //Query to update the password
                $password_update_query = mysqli_query($connect, "UPDATE users SET password='$md5_newpassword' WHERE email='$username'");
                echo "Your password has now changed!";

                }
                else{

                    echo "Your new password and re entered password does not match. Please try again.";
                    }
            }
            else {
                echo "Your old password does not match. Please try again.";
                }
        };

    }


?>
<div class="container">
<h3> Change your Password: </h3>
<form action="" method="POST" enctype="multipart/form-data">
    <div class="form-group">
      <label for="oldpassword">Old Password:</label>
      <input type="oldpassword" class="form-control" name="oldpassword" placeholder="Enter old password" >
    </div>
    <div class="form-group">
      <label for="newpassword1">New Password:</label>
      <input type="newpassword1" class="form-control" name="newpassword1" placeholder="Enter new password" >
    </div>
    <div class="form-group">
      <label for="newpassword2">New Password:</label>
      <input type="newpassword2" class="form-control" name="newpassword2" placeholder="Re-Enter new password" >
    </div>
     <center>
    <button type="submit" class="btn btn-primary" name="change_pass_submit" style=" background-color:#337AB7; color:white;">Change Password</button>
    </center>

 </form>
 </div>