Using the DateTime
class:
$a = new DateTime('now');
$b = new DateTime('2013-12-16');
$a->diff($b);
returns
DateInterval Object
(
[y] => 0
[m] => 1
[d] => 24
[h] => 8
[i] => 3
[s] => 23
[invert] => 0
[days] => 54 // This is what you're looking for?
)