PHP in Events

gistfile1.txt

<?php 
    $_GET['cal_id'] = (isset($_GET['cal_id'])) ? $_GET['cal_id'] : '';
    $_GET['event_id'] = (isset($_GET['event_id'])) ? $_GET['event_id'] : '';
?>

<?php if( empty($_GET['cal_id']) ) : ?>
    {!-- the links from the old site didn't use cal_id or event_id. this will preserve their functionality  --}
    {if segment_3}
        {exp:calendar:calendars calendar_name="{segment_3}" }
            {redirect="events/detail/{calendar_url_title}?cal_id={calendar_id}"}
        {/exp:calendar:calendars}
    {/if}
<?php elseif( empty($_GET['event_id']) ) : ?>
    {!-- preserve the old links and enable the "recommended classes" to work without the event_id --}
    {exp:calendar:events calendar_id="{get:cal_id}" date_range_start="today" date_range_end="1 year"}
        {occurrences limit="1"}
            {redirect="events/detail/{calendar_url_title}?cal_id={calendar_id}&event_id={event_id}"}
        {/occurrences}
    {/exp:calendar:events}
<?php endif; ?>