RewriteEngine on

RewriteBase /

# only if the requested file does not exists
RewriteCond %{REQUEST_FILENAME} !-f

# Store the current location in an environment variable CWD
RewriteCond $0#%{REQUEST_URI} ([^#]*)#(.*)\1$
RewriteRule ^.*$ - [E=CWD:%2]

# Just by prefixing the environment variable, we can safely rewrite anything now
RewriteRule ^([^/]*) %{ENV:CWD}index.php?database=$1 [QSA,L]