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.