gymrooster_php/php/dashboard.php

<?php

session_start();

require 'conn.php';

$result = $conn->query("SELECT * FROM docenten WHERE username = '" . $_SESSION['username'] . "';");

$docent = $result->fetch_assoc();

if (isset($_GET['date'])) {
  $date = date("Y-m-d", strtotime($_GET['date']));
} else {
  $date = date("Y-m-d"); // Create date in mysql DATE format.
}

$rooster = $conn->query("SELECT * FROM rooster WHERE docent = " . $docent['id'] . " AND dag = DATE('" . $date . "')")->fetch_assoc();