how to work with #_BOOKINGFORMCUSTOM in php?

dear forum,

may somebody has an answer for me:
i defined a #_BOOKINGFORMCUSTOM{id} field as a radiobutton, where my customers can choose “yes” or “no” or leave it blank and decide later (at the event).

if “yes”, i like to add a surcharge to the ticketprice (for the calculation itself i already have a script), but for “no” or no decision the surcharge should not be added – so i have to implant any if/then/else construct to the php and i am a bloody beginner in this ??

how can i work with the custom booking form field id in php?

thanks in advance ??
andreas

The page I need help with: [log in to see the link]