gymrooster_php/php/index.php

<?php

require "conn.php";

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

$result = $conn->query("SELECT * FROM rooster INNER JOIN docenten ON rooster.docent = docenten.id WHERE dag = DATE('" . $date . "');");

if ($result->num_rows === 1) {
  $rooster = [$result->fetch_assoc()];
} else {
  $rooster = $result;
}