如何在 PHP 7 中从页面重定向到页面？

Question

我正在用 PHP 7 和 HTML 5 制作一个电子商务网站。当我输入登录详细信息时，admin.php 只是刷新并且不会重定向到 index.php，因为它应该。我已经尝试过使用实际的 cookie，但出于安全原因我更愿意只使用会话 cookie。ADMIN LOGIN.php CODE<?phpsession_start();if(isset($_SESSION["manager"])){  header("location: index.php");  exit();}?><?phpif (isset($_POST["username"])&&isset($_POST["password"])){  $manager = preg_replace('#[^A-Za-z0-9]#i', '', $_POST["username"]);  $password = preg_replace('#[^A-Za-z0-9]#i', '', $_POST["password"]);  include "../storescripts/connect_to_mysql.php";  //$sqlquery =  $sql = mysqli_query($con, "SELECT id FROM admin WHERE username='$manager' AND password='$password' LIMIT 1");  $existCount=mysqli_num_rows($sql);  if($existCount == 1){    while($row = mysqli_fetch_array($sql)){      $id = $row["id"];    }    $_SESSION["id"] = $id;    $_SESION["manager"] = $manager;    $_SESSION["password"] = $password;    header("location: index.php");    exit();  } else {    // code...    echo 'Invalid Log In Credentials<br><br>';    echo'<a href="index.php">Click Here To Re-Enter Credentials</a>';    exit();  }}?>INDEX.php CODE<?phpsession_start();if (!isset($_SESSION["manager"])){  header("location: admin_login.php");  exit();}$managerID = preg_replace('#[^0-9]#i', '', $_SESSION["id"]);$manager = preg_replace('#[^A-Za-z0-9]#i', '', $_SESSION["manager"]);$password = preg_replace('#[^A-Za-z0-9]#i', '', $_SESSION["password"]);include "../storescripts/connect_to_mysql.php";$sql = mysqli_query($con, "SELECT * FROM admin WHERE id='$managerID' AND     username='$manager' AND password='$password' LIMIT 1");$existCount = mysqli_num_rows($sql);if ($existCount == 0){  header("location: ../index.php");  exit();}?>一旦设置了会话 cookie，我希望页面重定向到 index.php，但它没有这样做。