PHP form validation and submit to paypal for payment

can someone please help me on this :)

I have a form to submit a classified ad. The form uses PHP for validation. In the form there is a drop down menu where one can select its payment according to the display time of the ad.

I don't know how to keep the validation in place and only send to Paypal if everything is ok. Is it possible to do it in one step?

Or do I have to redirect the user to a confirmation page and there place my buy now button? Also I would like to 开发者_开发技巧use IPN to update the info in the DB. Thanks Max

The PayPal IPN document describes how you should do it.