I would like to modify the code that is run when the user hits the sign in button after filling out the username and password inputs. This would mean the first file that is called when the button is pressed. i need access to the data passed from the form, right before the database is checked if the username and password are accurate. 

Can anyone help me and tell me where the file is, and how i can get the variables of username and password?

thank you so much!