using Session ID to order something - php

Question

So I am making a project where you can order food at school cafeteria, I already have a project where you can make a order with the menu and the student, and time.
But now I want that the student can order at it's own and not select any student so for the order it takes the currently logged in student. Sorry for bad english and thanks for every answer, i appreciate it.
[enter image description here](https://i.stack.imgur.com/NFz6G.png)
So Schueler is student and Menu is menu
uhrzeit is time
Now I want that there is no student to select, it takes the session id to make a order.
This is my order/create.php file
<?php
$title = "Menü reservieren";
include '../layouts/top.php';
session_start();
if(!isset($_SESSION['schuelerid'])) {
die('Bitte zuerst einloggen');
}
//Abfrage der Nutzer ID vom Login
$userid = $_SESSION['schuelerid'];
$email = $_SESSION['email'];
echo "Hallo Schueler: ".$userid;
echo " ".$email;
require_once('../../models/Bestellungen.php');
// create new/empty model, contains all input-values and error-messages
$r = new Bestellungen();
if (!empty($_POST)) {
$r->setMenu(isset($_POST['menu']) ? $_POST['menu'] : '');
$r->setUhrzeit(isset($_POST['uhrzeit']) ? $_POST['uhrzeit'] : 0);
$r->setSchueler(isset($_POST['schueler']) ? $_POST['schueler'] : 0);
// redirect to view-page after successfull saving
if ($r->save()) {
header("Location: view.php?id=" . $r->getId());
exit();
}
}
?>
<div class="container">
<div class="row">
<h2><?= $title ?></h2>
</div>
<form class="form-horizontal" action="create.php" method="post">
<div class="row">
<div class="col-md-1"></div>
<div class="col-md-2">
<div class="form-group required">
<label class="control-label">Schueler *</label>
<select style="width: 200px" name="schueler">
<option selected></option>
<?php
require_once "../../models/Schueler.php";
foreach (Schueler::getAll() as $g){
?>
<option value='<?=$g->getId()?>'><?=$g->getName()?></option>
<?php
}
?>
</select>
<?php if (!empty($r->getErrors()['schueler'])): ?>
<div class="help-block"><?= $r->getErrors()['schueler'] ?></div>
<?php endif; ?>
</div>
</div>
<div class="col-md-2">
<div class="form-group required ">
<label class="control-label">Menu *</label>
<select name="menu" style="width: 200px">
<option selected></option>
<?php
require_once "../../models/Menu.php";
foreach (Menu::getAll() as $z){
?>
<option value='<?=$z->getId()?>'><?=$z->getName()?></option>
<?php
}
?>
</select>
<?php if (!empty($r->getErrors()['menu'])): ?>
<div class="help-block"><?= $r->getErrors()['menu'] ?></div>
<?php endif; ?>
</div>
</div>
</div>
<div class="row">
<div class="col-md-2">
<div class="form-group required ">
<label class="control-label">Uhrzeit *</label>
<input type="datetime-local" class="form-control" name="uhrzeit" value="">
<?php if (!empty($r->getErrors()['uhrzeit'])): ?>
<div class="help-block"><?= $r->getErrors()['uhrzeit'] ?></div>
<?php endif; ?>
</div>
</div>
<div class="col-md-1"></div>
</div>
<div class="row">
<div class="form-group">
<button type="submit" name="submit" class="btn btn-success">Erstellen</button>
<a class="btn btn-default" href="view.php">Abbruch</a>
</div>
</div>
</form>
</div> <!-- /container -->
<?php
include '../layouts/bottom.php';
?>