what I would do: (using php too)
1. create another column - newdate, or whatever. give it the type datetime or date, whatever you need.
2. run a query getting all your dates and primary key fields out into an array
3. foreach through your results, reformat the date using strtotime (with a bit of hacking around maybe to get the right format)
4. run an update for each iteration of the loop and put the new value in your newdate field
5. delete the old date column.
6. rename newdate as appropriate