这段代码总是说旧密码不正确 - This piece of code always says that the old password is incorrect?

This piece of code always says that the old password is incorrect?

我正在尝试创建一个密码更改工具，要求用户输入旧密码，新密码和确认新密码。如果旧密码与数据库匹配，则进行密码更新。否则显示旧密码不正确的声明。但是这段代码总是说旧密码不正确。有人能找到这段代码中的bug吗?当用户注册时，我使用md5过程来存储密码。这和这件事有关系吗?请原谅我的冗长代码。我还是个初学者。谢谢你:)

<div class="col-sm-4">
   <h1 class="register-title">Change Password</h1>
       <form action="<?=$_SERVER['PHP_SELF']?>" method="post" class="register">
       <input type="password" name="oldpassword" class="register-input" placeholder="Enter Old Password">
       <input type="password" name="newpassword" class="register-input" placeholder="Enter New Password">
       <input type="password" name="password2" class="register-input" placeholder="Confirm New Password">
       <input type="submit" name="submit1" value="Update Password" class="register-button">
       </form>
 </div>

PHP

<?php if (isset($_POST['submit1'])) { $oldpassword=$_POST['oldpassword']; $newpassword=$_POST['newpassword']; $password2=$_POST['password2']; //$password=md5($password); $connect = mysqli_connect('localhost', 'root', '', 'blood'); if (mysqli_connect_errno()) echo "Failed to connect to MySQL: " . mysqli_connect_error(); $query12 = "select password from users WHERE uid = '$uid'"; $query13 = "UPDATE users SET password='$newpassword' WHERE uid = '$uid'"; $result12=mysqli_query($connect,$query12) or die('Error, query failed'); if (mysqli_num_rows($result12) == 0) { echo 'Database is empty <br>'; } elseif ($result12) { while ($row = mysqli_fetch_array($result12)) { if ($oldpassword == $row["password"]) { $result13=mysqli_query($connect,$query13) or die('Error, query failed'); } else { echo "incorrect old password"; } } } }?>

如果您说您在存储密码时使用md5，那么您认为password将如何存储什么值?哈希值。

所以如果$row["password"]有散列值，而你的$_POST['oldpassword']是没有散列，那么你期望什么?
正确的比较方法应该是:

if (md5($oldpassword) == $row["password"])
顺便说一下，使用md5是不再安全