require_once

Hello, I've got probably a simple question on require_once. I've got a file that has require_once at the top of it pulling in another file of functions. Later on in this file I've got another require_once bringing in a second file. In this second file I have a function call to a function defined in the first files' top require_once functions file. I'm getting a call to undefined function. Should I change he require_once to require at the top of the first file to fix? I thought require_once meant it was in that file and anything pulled in later? Thanks. Dave.