Accessing fields from User Detail App

Hello,

I have created extra fields for user to fill when they checkout, using User Detail App, and now I want to access those fields values in code here in this file /public_html/dev/components/com_payplans/controllers/payment.php in function complete()

So for example if user fill those extra fields, i want to access those added values in above stated function and file (to use with my API calls to my accounting system). Please let me know how I can do that?

Thanks
Mohammad