How to find <form> section

i am checking some script & that’s in OOP i don’t know much about that want to edit some things anybody can help in finding out some object searching where <form> and other input fields are mention an define in this or in which file i need to edit where i can find form section section thanks
here is code file attach plzz check and help me

It doesn’t appear to be in that code, look in the output code for the exact form tag and do a mass file-search for it.

Off Topic:

That code is rather ugly, it could have been done so much neater. Did you buy it, or is it free code? Either way, you may have done it better if you wrote it yourself after looking into those payment APIs; Even if you couldn’t have done it better, or even as good, you would have understood what’s going on and that puts you in a much better place.