Well you can start with the wiki page: https://wiki.fogproject.org/wiki/index.php?title=Include_any_ISO_in_the_FOG_Bootmenu
And here is an older document, but the concepts still apply. There is no need to hack the php code any more since you can edit the pxe boot menu inside the fog web gui now. https://wiki.fogproject.org/wiki/index.php/Advanced_Boot_Menu_Configuration_options