<?php
    $sent_secret = $_GET["secret"];
    $config_file = file_get_contents("merge-calendars-config.json");
    $data = json_decode($config_file);

    if(is_null($sent_secret) || $sent_secret !== $data->secret)
        header('HTTP/1.0 403 Forbidden');
    else {
        $all_calendars = "";
        foreach ($data->calendars as &$calendar) {
            $ical = file_get_contents($calendar);
            $all_calendars = $all_calendars . $ical;
        }
        unset($calendar);

        $pattern = "/(\r?)\nEND:VCALENDARBEGIN:VCALENDAR/";
        $replacement = "";
        $all_calendars = preg_replace($pattern, $replacement, $all_calendars);

        $calendar_filename = "calendar.ics";
        file_put_contents($calendar_filename, $all_calendars);
        header("Content-disposition: attachment;filename=$calendar_filename");
        header('Content-Type: text/calendar');
        readfile($calendar_filename);
        
        ob_end_flush();
        flush();

        unlink($calendar_filename);
    }
?>