Create a spam-proof script for my PHP form, add PayPal redir

I have created a PHP web page with a form. The form has a drop down menu, a text area, a radio button group, a checkbox. You will do following: -write a contact form to mail script -validate fields (i.e. text area must be min 400 characters), must choose selection from drop down). Upon submit if errors show pop-up with errors. -upon submit send user to PayPal based on radio button selection (three different options, so three different prices). I will create products in PayPal account. -upon payment return them to website thank you page with unique order ID # -send form field results to admin email -send html formatted email to submitter