I want to write custom PHP code for a button, is it possible?

I want to write custom PHP code for my form, is it possible?

No, it is not possible to add PHP and Javascript codes inside the form builder and on your forms. However, this can be done if you'll use the form's full source code, here's a guide on how to get that code: https://www.jotform.com/help/104-How-to-get-the-Full-Source-Code-of-your-Form. You can also download the some source files of the form like CSS and Javascripts and host it in your own server. However, please note that changes on the form using the form builder will NOT reflect on your website where the form's full source code is embedded, you need to get a fresh copy of the full source code, apply the custom scripts you previously added and host the files again.

If you just want to run a PHP script and not directly add it on your form, you can do that using WebHook. Here's a guide about it: https://www.jotform.com/help/245-How-to-Setup-Webhook-with-JotForm 

I hope that helps.