So, I'm working with a really old system which uses a person's mysql database credentials to authenticate to a web site (the database was originally only accessed from the command line, but is now accessed from a php frontend). Because of some internal reasons (and to preserve the user's history), I have to leave the old authentication intact. I've been charged with adding openid authentication to this system. Somehow I need to be able to retrieve a users mysql username and password upon logging into the site through openid (using the Zend framework, by the way). I've thought of simply requiring registration at the first login, where the user must provide their mysql credentials, but I'd rather not store the password plain text. I've also considered blanking everyone's mysql passwords, and just setting the user's mysql username manually (rather than having the user provide this, since they could provide any username). This is turning into a security nightmare. Does anyone have any suggestions for alternatives?

This is running on a Linux server, by the way. Also, I can't use mysql pluggable authentication because the mysql version is 5.0 (pluggable authentication requires mysql 5.5), and no, I can't update it.