When it comes to php, nearly anything is
possible.
I use Nuke from time to time, my old site was with PHPNuke.
I did some custom work with it so that the "Content" pages (i'm talking module here) had extra fields under the "Signature" field. The fields were "Keywords" and "Description". Then you modify the database to accomidate the appropriate fields. I just input what I wanted for those fields like any other. I think I also had it set so that the title was automatically added to the keywords listed.
I set the pages title to be pulled from Nuke's "Title" field on the content page (know what I mean?...I think it just says "Content - SiteName by default").
Anyway, you can see what I mean at
www.new.divergentlines.com
Look at the pages and you'll see the titles and meta tags change.
To do this dynamically, you could make some php script to take every word over 3 letters in the first 100 charactors and stick them in the keywords tag. I don't exactly know the best way to do it, but I'm sure it's possible.
For a dynamic description you could also do something like "This page is about xxx. MySite is super cool." where "xxx" is the title variable. This would crank out very similar description tags for each page, but it would be done without the need to input extra data.
Let me know if you want to see how I modified the "Content" module's index.php.
Also, if you're new to Nuke, make sure you check out
www.nukecops.com I think it recently changed owners, but it used to have the best info about PHPNuke.
Make sure you secure the site. Because Nuke is so popular it is a big target for hacks; most sites share the same vulnerabilities. I posted my story a few months ago on another forum
here.
If I was unclear about anything, let me know.