how to make good use of includes

Because my functions tend to become rather lengthy I split up the
functions for each subject or action in a function (a) that checks user
input (filter and validation) and if this checks out ok it passes the
variables to another function (b) that retrieves or stores the data
from/in the database, function (a) then communicates the result (true,
false, error, list with data) to the client.
Currently I organized all functions per subject (all functions that
handle for example 'groups': get, add, modify, delete in a file) and I
include all the files when the webservice is called, this is over 500 kB.
A different approach would be to only include the functions needed, this
means a file for each function, so a lot more includes.

What is a good approach ?

Pugi!

> What is a good approach ?

Enter the 21th century and use classes + autoload?
Also, it's always a good idea to f'up2 existing newsgroups.