Hi 🙂
{I'm obviously new here also, and to php, so please don't beat me over the head if I have missunderstood your desires 🙂}
If I understand what you are wishing to be-able to accomplish then why not use symbolic linking, this works great under Unix/Linux/Freebsd etc. systems 🙂
Now if all sites/domain names reside on the same server and under the same tree,(even if they have different ip numbers & domain names for the outside world) then just pick a domain as the one that will hold all your data to be shared and create all the directories required and store all the common data under these directories and then with symbolic linking you create linked directories from your other domains to whatever directory you wish.
For instance:
You could make it that www.clientsite.com/bbs points directly to bbs.clientsite.com and then create all your other real directories under bbs.clientsite.com
This would mean both bbs.clientsite.com/images and www.clientsite.com/bbs/images would point to the same location and thus obtain the same data/images/whatever 🙂
For this all images would truly reside under bbs.clientsite.com/images but to all intents and purposes, even for ftp'ing they are also under www.clientsite.com/bbs/images 🙂
I run a few business websites that are contained under 1 common master account and directory on a virtual server and under this master account lie the domains and subdomains each with a unique IP address for the web in directory form... To create a link directory anywhere under this master account and link it to a domain is very easy, I have all 7 websites utilising a common images and coding repository and each site sees this respository as belonging to it's own domain so I can do things like "/images/whatever.jpg" etc. under each domain and they'll all utilise the same data 🙂
It's great for saving on space and duplication of effort also 😃
If bbs above didn't exist you could have done this while in the www.clientsite.com domain,(directory on your system).
ln -s /htdocs/users/youraccountname/youraccountname/bbs.clientsite.com bbs
[my host has me with a double accountname like above, dunno why, think it's part of the added security setup since they allow SSH.]
The above would create a directory that is seen by the system as a real directory, but which is really pointing/redirecting all requests from the web or command line for www.clientsite.com/bbs to bbs.clientsite.com
To get all that htdocs etc information, just go to your base directory, 1 directory before all your domains & subdomain names show up as directories and do a pwd command to show the full path to this base directory,(you could probably grab this info from your ftp program also maybe).
Hope this helps 🙂
If not... then after all this writing, just pretend it did.. lol 😃
Later RIPP.