You'd have to try and see...
If you store the date as a date-time type, the database will probably be faster, because it's date_format() function is built to deal with just that type of data.
A PHP function would have to read the string returned by the database, and re-arrange it,
or you'd have to make the database give you a timestamp that strftime etc can handle.
But like I said, try and see which is faster.