更改密码代码无效

Question

我正在为我的网站制作一个密码更改程序，这导致我的所有用户密码都变成了相同的密码。

我使用的代码如下所示。

如果有人能帮助我，那将非常感谢他/她。

//if form has been submitted process it
<br/>
if(isset($_POST['submit'])){
    $stmt = $db->prepare('SELECT password FROM user WHERE password = :hashedpassword');
    $stmt->execute(array(':hashedpassword' => $_POST['password']));
    $row = $stmt->fetch(PDO::FETCH_ASSOC);

    if(strlen($_POST['password']) < 3){
         $error[] = 'Password is too short.';
    }

    if(strlen($_POST['passwordConfirm']) < 3){
    $error[] = 'Confirm password is too short.';
}

if($_POST['password'] != $_POST['passwordConfirm']){
    $error[] = 'Passwords do not match.';
}


//if no errors have been created carry on
if(!isset($error)){

    //hash the password
    $hashedpassword = $user->password_hash($_POST['password'], PASSWORD_BCRYPT);

    try {

        //insert into database with a prepared statement
        $sql="UPDATE user SET password = :hashedpassword";
        $stmt = $db->prepare($sql);
        $stmt->execute(array(
            ':hashedpassword' => $hashedpassword                                
        ));

        //redirect to index page
        header('Location: login.php?action=resetAccount');
        exit;

    //else catch the exception and show the error.
    } catch(PDOException $e) {
        $error[] = $e->getMessage();
    }

}

}