Your best bet would be to implement htaccess rewrites for each subdomain in order to redirect to another site’s html if that’s what you want to do.
On optimisation, if your engine is creating the user pages on the fly, have it create a static XHTML index.html for each user account and implement lighttpd (or similar) as your webserver rather than the ever-hungry Apache.
Depending on how you’re going about this, the engine account could easily be set up to have write privileges on the sub-domain directories, so if a user decides they would like a different look to their page (have your developer use a CSS repository) they can login to your main “engine” site and choose a new design - and any textual/picture adjustments they’d like to update - and the engine would then create and overwrite their index.html on submission, giving them an instant transformation. Extra carrot to wave at them, no extra cost to you.
Only your engine site would need PHP or PERL plus database access, the subs being completely static, which will both improve performance dramatically across the entire account and see your user’s pages delivered to browsers within milliseconds.
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