无法修改标头信息-标头已发送，并且没有echo语句

Question

我收到“无法修改标头信息-标头已由...发送”警告，并且标头功能之前没有echo语句。

<?php
$error = "";
if (isset($_POST['submit'])) {
  try{
      $conn = new PDO(DSN,USERNAME,PASSWORD,$options);
      $conn->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
      $email = $_POST['email'];
      $password = $_POST['password'];
      $loginQuery = "SELECT email,password FROM users WHERE email = :users_email AND  password = :users_password";
      $statement = $conn->prepare($loginQuery);
      $statement->execute(array(":users_email"=>$email,":users_password"=>$password));
      $result = $statement->fetchAll();
      if($result){

      }
      else
        {
          header("Location: pages/signup.php");
        }
    }catch(PDOException $ex ){`
      $error= "error occured". $ex->getMessage();
    }

}