You should write those function in common_functions file which are common to your project.

Suggestion: Don't keep you database/validation function in this file - This will mess with your functionality road-ahead.

Functionality:
------------
1. Login script
2.Validate logins
3.reports oriented queries as they are required now an then.
and many more which you can better jugde based on your projects

Logs:
-----
1.create log
2.update log
3.archive logs

Mailing
-------
mails

Session
--------
check session expire - This will be used everywhere so you can add it in common_functions

session creation code should be in your constant defining file not in common function file.

Thanks.