How to calculate difference of two string dates in days using PHP?

Question

I have two date variables:

$dnow = "2016-12-1";
$dafter = "2016-12-11";

I want to calculate the difference of this two dates which are in string format so how do I calculate? I used

date_diff($object, $object2)

but it expecting two date object, and I have dates in String format , After using date_diff I get following error

Message: Object of class DateInterval could not be converted to string.