Creating a master user database

First of all, this isn't ready for a bid, as I'm not sure what the scope of this will be, or if it is even feasible. I don't have a huge budget for this, so to avoid wasting anyone's time, I'd like to get some input from you before I decide to proceed. I have a number of "off the shelf" scripts all of which have their own user tables. with usernames, passwords, email addresses, etc. What I want is to have a new "master" users database that I can use with all of them, as well as to be able to add more information like name, phone, address, etc. The other scripts would look to this new database for the username and password. The individual scripts would continue to control user's access to the features of each script, as well as continuing to set individual cookies and sessions. Don't know if this is a job for LDAP, or adding an include to each individual script so it looks for user info on the master database. Just looking for the most efficient and cost-effective approach to this. What do you think?