数据库中的密码字段更改为 0

此代码不会更改已登录用户的密码，而是更改为 0，然后不允许我重新登录。我知道 md5 不是最安全的，因为这不会只是项目的活动站点。但是，我愿意接受有关替代方案的建议。我在数据库中有两个字段，称为密码和密码2，一旦他们更改密码，就需要更改这两个字段。此外，错误消息不会显示。

<?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);
                    $md5_newpassword2 = md5($newpassword2);
                    //Query to update the password
                    $password_update_query = mysqli_query($connect, "UPDATE users SET password='$md5_newpassword' AND password2='$md5_newpassword2' 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>