Quick Question

I am writing a plugin and would like to keep the whole thing on a single file. I have already written all the various forms that would appear depending on the choices you make in the sub menus. Now what I would like to know is can you call a certain function (function to process the data given by user in the form) in the same file to process the form data when a person clicks the submit button.

Thanks in advance for any and all help with this. I am new to php coding and I am having fun trying to write this plugin , I just want to keep it to a single php file.