登录后会话验证无效

我已经构建了一个验证用户的php登录，如果成功则重定向到landing.php . 否则，将向用户提供登录错误 . 一旦登陆landing.php，我试图检查$ _SESSION是否存在 . 如果是的话，那就用一块吧 . 如果不是，则应将用户重定向到登录页面 . 但是，如果我直接在浏览器中键入landing.php url以跳过登录，则即使$ _SESSION为空，页面也不会重定向 . 请告知检查 Session 的最佳方式 .

我的代码：

landing.php :( PHP脚本）

<?php
session_start();

  include('inc.php');

  if (!(session_id())) {

    header("Location: ['url']/index.php"); /*url not displayed for security*/

  } else {
    $welcome = "Welcome ".$_SESSION['userinfo']['firstname'];
  }

?>

login.php :(创建会话的地方）

<?php
session_start();

include("inc.php");


if ((isset($_POST['username'])) and (isset($_POST['password']))) {

    $login = $_POST['username'];
    $pass = $_POST['password'];

} else {
    //echo json_encode("Credentials did not save. Please try again.");
    echo "Credentials did not save. Please try again.";
}


$conn = new mysqli($servername, $username, $password, $dbname);


if ($conn->connect_errno) {
    //echo json_encode("There was a problem connecting to MySQL: (" . $conn->connect_errno . ") " . $conn->connect_error);
    echo "There was a problem connecting to MySQL: (" . $conn->connect_errno . ") " . $conn->connect_error;
    //echo json_encode("There was a problem connecting to MySQL.");
}


$sql = "SELECT * FROM sched_users WHERE login = ? ORDER BY login LIMIT 1";

if (!($sql = $conn->prepare("SELECT * FROM sched_users WHERE login = ? ORDER BY login LIMIT 1"))) {
    //echo json_encode("Prepare failed: (" . $conn->errno . ") " . $conn->error);
    echo "Prepare failed: (" . $conn->errno . ") " . $conn->error;
    //echo json_encode("Prepare failed.");
}


if (!($sql->bind_param("s", $login))) {
    //echo json_encode("Binding parameters failed: (" . $sql->errno . ") " . $sql->error);
    echo "Binding parameters failed: (" . $sql->errno . ") " . $sql->error;
    //echo json_encode("Binding parameters failed.");
}


if (!$sql->execute()) {
    //echo json_encode("Execute failed: (" . $sql->errno . ") " . $sql->error);
    echo "Execute failed: (" . $sql->errno . ") " . $sql->error;
    //echo json_encode("Execute failed.");
}


$res = $sql->get_result();


if ($res->num_rows === 0) {

    //echo json_encode("No user record found.");
    echo "No user record found.";

} else if (!(($res->num_rows === 1) or ($res->num_rows === 0))) {

    //echo json_encode("Too many results returned.");
    echo "Too many results returned.";

} else {

    $row = $res->fetch_array();

    $hash = $row['password'];

    if (password_verify($pass, $hash)) {

        /*
        $session = true;
        echo json_encode($session);
        */

        $_SESSION['userinfo']['firstname'] = $row['firstname'];
        $_SESSION['userinfo']['lastname'] = $row['lastname'];
        $_SESSION['userinfo']['email'] = $row['email'];
        $_SESSION['userinfo']['phone'] = $row['phone'];
        $_SESSION['userinfo']['company'] = $row['company'];
        $_SESSION['userinfo']['department'] = $row['department'];
        $_SESSION['userinfo']['admin'] = $row['admin'];
        $_SESSION['userinfo']['statusflag'] = $row['statusflag'];
        $_SESSION['userinfo']['revoked'] = $row['revoked'];


        header("location: [url]/landing.php");/*url not displayed for security*/

    } else {
        //$session = false;
        //echo json_encode($session);
        //echo json_encode("The username or password does not match. Please try again.");
        echo "The username or password does not match. Please try again.";
    }

}


$sql->close();
$conn->close();

?>