Quick fix for a web application required ASAP

PHP programmer with experience of developing JavaScript and Ajax applications needed ASAP to update an existing web application which is currently live.

To summarize, we have a cost calculator on our website which enables students to work out the price of a particular course and accommodation (if required). The utility queries a database to check the availability of a course based on the dates the school is closed. Please see COURSE PLANNER using the following web link to see how it works:

http://celt.co.uk/index.php/en/contacts-and-bookings/celt-calc
(please scroll down)

Our application needs to be updated as soon as possible to prevent people from booking certain courses on certain dates. Some course dates are full while others have more availability but we really need to prevent people from booking dates which are already fully booked.

We would need a quick fix to update the code so that every time a different course is selected, the database is queried to update the list of dates available for that particular course.

For someone with experience of using PHP, JavaScript, Ajax this should be relatively straightforward. More information will be provided later. Can you help us??