You should be using DateTime Object for these operations
$joinDate = "2014-05-26";
$joinDate_obj = new DateTime($joinDate);
$now = new DateTime();
$interval = $joinDate_obj->diff($now);
$diff = $interval->d ;
echo $diff; //12
The object $interval will have 
DateInterval Object (
  [y] => 0 
  [m] => 11 
  [d] => 12 
  [h] => 12 
  [i] => 49 
  [s] => 4 
  [weekday] => 0 
  [weekday_behavior] => 0 
  [first_last_day_of] => 0 
  [invert] => 0 
  [days] => 347 
  [special_type] => 0 
  [special_amount] => 0 
  [have_weekday_relative] => 0 [have_special_relative] => 0 
) 
So you may use for example $interval->days for the difference in days.