If you have multiple people working on the same files you will want to use a revision control system like SVN, GIT, Mercurial or even CVS and then merge changes at the end of the day:
Revision control, also known as version control and source control (and an aspect of software configuration management), is the management of changes to documents, programs, large web sites and other information stored as computer files. It is most commonly used in software development, where a team of people may change the same files. Changes are usually identified by a number or letter code, termed the "revision number", "revision level", or simply "revision". For example, an initial set of files is "revision 1". When the first change is made, the resulting set is "revision 2", and so on. Each revision is associated with a timestamp and the person making the change. Revisions can be compared, restored, and with some types of files, merged.
As for
Is there a way to include methods into a class using the include() function?"
see my answer to