given those two date strings, you could convert to timestamps like so:
list ($start_day, $start_month, $start_year) = explode ('/', $startdate);
list ($event_day, $event_month, $event_year) = explode ('/', $eventdate);
// 86400 is the number of seconds in a day
$diff = (mktime (0,0,0,$event_month, $event_day, $event_year) - mktime (0,0,0,$start_month, $start_day, $start_year)) / 86400;
You could avoid using mktime() by simply using strtotime() instead, but I'm not sure that strtotime() will handle those european date formats (dd/mm/yy) correctly. I think it expects mm/dd/yy.
