使用值进行测试，它可以工作，但是当我放置变量时，它不会显示文字

Question

  <?php 
    $ime_prezime="";
    $email="";


    $con = mysqli_connect("localhost","root","","tvz");
    $result1=$con->query("SELECT naziv FROM mentori");
    $result2=$con->query("SELECT ime_prezime FROM studenti");

<form action="index.php" method="POST">

        <h1>Izbor mentora</h1>
        <div class="text-center">
        <h2>Mentor</h2>
        <select class="chosen" name="mentor">
            <?php 
            while($rows=$result1->fetch_assoc())
            {
                $ime_prezime=$rows['naziv'];
                echo "<option value='$ime_prezime'>$ime_prezime</option>";
            }


            ?>


        </select>
        <h2>Vase ime</h2>
        <select class="chosen" name="student">
            <?php 
            while($rows=$result2->fetch_assoc())
            {
                $email=$rows['ime_prezime'];
                echo "<option value='$email'>$email</option>";
            }


            ?>


        </select>
        </div>
        <input type="Submit" class="ok" name="register_button" value="Update">
        </form>
    <?php 
      $_SESSION['ime_prezime'] = $email;
      $_SESSION['naziv'] = $ime_prezime;
      if(isset($_POST['register_button'])){
        $id = mysqli_query($con, "SELECT id FROM mentori WHERE 
         naziv=$ime_prezime");
        echo "$id";
        $query = mysqli_query($con, "UPDATE studenti SET id_mentor='$id' WHERE 
         ime_prezime='ivanic'");
    }
     ?>

我已添加所有内容，但不会应用到我的数据库中