So you want something that goes something like:
$input_date='12 December, 1999';
$output_date = date('Y-m-d', strtotime($input_date));
echo $output_date;
The Right Thing would be for that to work, of course, but English is a bit fuzzier than strtotime() can cope with. The problem is the ,, of all things.
$input_date=str_replace(',', '', '12 December, 1999');
$output_date = date('Y-m-d', strtotime($input_date));
echo $output_date;
where, needless to say, '12 December, 1999' is to be replaced with the real source of the date.