Question about include_once and require_once

The php lit is not clear, to me, exactly what is happening. Does the "once" refer to the session or to the page? In other words: suppose I have a file with several functions and several pages on my site. My visitors can arrive at any page and then navigate around the site. If I use include_once () on all my php pages, can I rely on the session manager to maintain the functions for the visitors? If I use just plain "include ()", does the php parser load up an additional copy for every time it's called? Thanks....