PHP中的登录表单验证和错误消息

我有一个需要用户名和密码的登录表单。如果登录凭据错误，我希望表单顶部显示"无效密码"或"无效用户名"。有人能提供一些见解吗？

如果一个字段当前为空，则消息显示"无效密码"。我希望它能有信息，即使领域里有什么东西是错误的。

这是登录表格：

 <form action="index.php?action=login" method="post">
                    <fieldset>
                        <div style="color:red;"><?php echo isset($_REQUEST['err']) && $_REQUEST['err'] == 1 ? "Invalid     Password" : "";?></div>
                        <legend>Login</legend>
                        <label for="loginName" class="required">Username:</label>
                        <input id="loginName" name="loginName" type="text"
                               value="" required />
                    <label for="password" class="required">Password:</label>
                        <input id="password" name="password" type="password"
                               value="" required />
                        <input id="submit" class="submit" type="submit" value="login"/>
                    </fieldset>
             </form>

这是登录功能（用于会员/管理员网站，因此登录到两个帐户）：

 function connect($loginName) {
    global $db;
    $query = "SELECT email, level, password FROM members WHERE email = '$loginName'";
    $result = $db->query($query);
    $results = $result->fetch(PDO::FETCH_ASSOC);
    return $results;
 }
//Login
function login($loginName, $password) {
    $results = connect($loginName);
    if(!$results) {
        header('Location: /tire/admin/home.php?err=1');
    }
    if ($loginName === $results['email'] && password_verify($password,$results['password'])) {
        $_SESSION['loginName'] = $loginName;
        if ($results['level'] === 'a') { // 1 == Administrator
            $_SESSION['level'] = 'Administrator';
            header('Location: /tire/admin/home.php');
         } elseif ($results['level'] === 'm') { // 1 == Member
            $_SESSION['level'] = 'Member';
         header('Location: /tire/member/home.php');
         exit;
         }
     }
     header('Location: /tire/admin/home.php');
  }
//Logout
 function logout() {
    $_SESSION = array();
    session_destroy();
 }

@巴克里亚瓦德：这就是我尝试你的建议的地方，但仍然不起作用。它告诉我$loginName和$password是未定义的索引。

function error_message(){ unset($error); 
    $loginName = $_SESSION['loginName'];
{$results = connect($loginName);
    $loginName === $results['email'];
 $password = password_hash($_POST['password'], PASSWORD_BCRYPT);
  $passwords = password_verify($password,$results['password']);
   if(!$results) {$error = "Username not found";} //if no records returned,      set error to no username
   else //if found     {
     if ((isset($password)) !== (isset($passwords)))  //check password, if matched log him in
     { $error = "Password is wrong"; } //if not matched then set error message
   }
 }
   if(isset($error))echo $error; //if there is an error print it, this can      be anywhere in the page
 }