<?php
    session_start();
    $teachername="";
    $classname="";
    $classoutline="";

    //if user is not logged in, they cannot access this page
    if(empty($_SESSION['account'])) {
        header('location:../index.php');
    }    

    //connect to the database
    $db =  mysqli_connect('localhost','root','','registration');
    $db->set_charset("utf8");


    if ($db->connect_error){
        die("連接失敗".$mysqli->connect_error);
    }
    else {
        echo "資料庫伺服器連結成功及資料庫開啟成功<br>";
    }

    $teachername = $_POST['teachername'];
    $classname = $_POST['classname'];
    $classoutline = $_POST['classoutline'];


    //get teachers uid
    $result = $db->query("SELECT id from users where account = '".$_SESSION['account']."'");

    while ($row = $result->fetch_assoc()) {
        $uid = $row['id'];
    }

    //get course t_id
    $newresult = $db->query("SELECT tID from teachers where uid = $uid");

    $newrow = $newresult->fetch_assoc();
    if($newrow){
        $t_id = $newrow['tID'];
    }


    // teachers and course insert to db-table

    //teachers
    $sql = "INSERT INTO teachers (tname,uID) VALUES ('$teachername','$uid')";

    if($db->query($sql)){
        echo "編號".$t_id."老師新增成功<br>";
    }else{
        echo"老師新增失敗<br>";
    }

    //course
    $newsql = "INSERT INTO course (cname,coutline,t_ID) VALUES ('$classname','$classoutline','$t_id')";

    if($db->query($newsql)){
        echo "課程新增成功";
    }else{
       echo "課程新增失敗";
    }
?>

想請教一下大家!
現在正在做一個學習網的專題，資料庫有會員、老師、課程、學生，4張表
因為每位會員都可以進行開課、上課，所以有用外鍵去連結每個表
現在已經用好了"老師資料表u_id"與"會員資料表id"的關聯，但在"課程t_id"、"老師tid"就遇到一些問題
$t_id可以echo出來，但是沒辦法加進課程的資料表 不知道是哪裡有問題...