Simple Form in PHP with some data

I have a PHP form I have this form , it sends me the email with the fields ive already confogured i want some fields to hace some special functions i want the start and end date to display the difference between dates when dates are picked up. so it says X number of days i have a dropdown menu , with some options i want to define each field a price and then multiply it for the number of dates in example single 100$ double 200$ tiple 300$ cuad 400$ if people picks up single , from 11/11/11 to 12/11/11 it will display (1 Day) then people will pick from single double or triple and it will be multiplyied for the number of days if people selechs from the option called paypal and put send the form will send client to paypal , if payment is sucessfull it will send a thank you message and trigger an email for me and the client. if client pick other payment form it will send a different thank you message and send the client and me an email containing the form data