在php mysql中登录会话错误

Question

我不擅长php，但我试图在每个页面上进行会话检查。 但每次我将existsCount作为值！= 1我的意思是不正确的信息。

    <?php   
      session_start();
      if(isset($_SESSION["admin"]))
      {
        header("location: nits-admin/$pagename");
        exit();
      }
      if (isset($_POST["username"])&&isset($_POST["password"]))
      {
        $admin = preg_replace('#[^A-Za-z0-9]#i','',$_POST["username"]);
        $password = preg_replace('#[^A-Za-z0-9]#i','',$_POST["password"]);
        include(dirname(__FILE__) . '../nits-include/db.php');
        $sql = mysqli_query("SELECT * FROM nits_users WHERE username ='$admin' 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;
            $_SESSION["admin"] = $admin;
            $_SESSION["password"] = $password;
            header("location: nits-admin/$pagename");
            exit();
        }
        else
        {
            echo 'Information incorrect';
        }
       }

     ?>