Abfrage nach Jahr/Monat

$query = "SELECT id, event_enddate, event_startdate, guest, gruppe FROM tbl_entries ORDER BY gruppe, event_startdate desc";
$result = mysqli_query($conn, $query);
$output = '';
if ($result && $result->num_rows > 0) {
    // initialize array to manage archived events
    $data = array();
    // add each archived entry to $data[] as dimensional array entry
    while ($row = $result->fetch_assoc()) {
        $year = date('Y', strtotime($row['event_startdate']));
        $month = date('F', strtotime($row['event_startdate']));
        $group = $row['gruppe'];
        $data[$year][$month][$group][] = $row;
    }
    $result->free();
    // iterate through years; generate subheading for each year
    // output months
    foreach ($data as $_year => $_months) {
        $output .= "<h2>{$_year}</h2>";
        foreach ($_months as $_month => $_groups) {
            $output .= "<h3>{$_month}</h3>";
            foreach ($_groups as $_group => $_entries) {
                $output .= "<h4>{$_group}</h4>";
                $output .= "<ul>";
                foreach ($_entries as $_entry) {
                    $output .= '<li>'.$_entry['event_startdate']." ".$_entry['event_enddate']." ". $_entry['guest'].'</li>';
                }
                $output .= "</ul>";
            }
        }
    }
    echo $output;
}