I am currently migrating my server configuration away from mod_php towards mod_fcgid (the successor of mod_fastcgi), as this allows me to use different users for executing scripts in different directories. I use this to have every hosted virtual domain using its own system user. This should (in theory) prevent one buggy application to take over all other hosted domains as well.
I though faced one problem: I could not get phpMyAdmin working and this was a requirement of one of my clients. phpMyAdmin kept popping up the authentication dialog over and over again when using HTTP Basic Authentication.
After searching some time, I noticed that, when using PHP in CGI mode, the authentication data is not passed over to the script by default. A FAQ entry of phpMyAdmin brought the solution to this issue: a ReWrite Rule was needed for the directory containing phpMyAdmin:
RewriteEngine On
RewriteRule .* - [E=REMOTE_USER:%{HTTP:Authorization},L]
Suddenly phpMyAdmin worked 😉