This can be done via plugins/code then you call your plugin with: plug.php?r=myplugin
..but what is more important...
I got a site with a lot of ajax stuff and the current common.php istn practical at all.
Sometimes I need access to DB only, another time I need DB + user rights only.
The biggest problem is the ob_start('ob_gzhandler'); - you cant redo the gzip compression if not needed (eg, when outputting big JPG files it takes years to compress it)
My idea is to split common.php file into 3 if - else parts - driven by an GET param. Right now I use external php files with only parts from common.php - really odd :p
Perl - the only language that looks the same before and after RSA encryption.