Time input formatting in PHP-12 hour clock

Hi, All-- Has anyone already solved the problem of how to get a time input from the user in people time (12-hour clock) and format it to be INSERTed in a mySQL time field? The mySQL time field is 24-hour and of course it can be formatted in the SELECT statement to be displayed as 12-hour for the user. But--presumably in PHP--I need the user to be able to input a time, then validate it as to whether it is complete (e.g., expresses AM or PM and evaluates correctly), then pass it into an INSERT statement into the time field of a database. I suspect this already exists, just can't find an example. TIA Dave Shugarts