If a user visits www.example.com/forum could this point to another server/IP without changing the actual URL the user sees?
This means that every request after that must pass to the other server while sending all the post/get data so that www.example.com/forum/index.php?some=thing must work as if it was myothersite.example.com/forum/index.php?some=thin
edit: all this is because the registrant doesn’t allow to manage DNS records or even create subdomains.
Nope, this is not possible – you’d have to make Apache act as a Proxy for this to work, and this brings other problems:
All traffic would have to run through both sites
On shared hosting, you probably won’t be allowed to do this
REMOTE_ADDR, cookies and other things would no longer work as expectedthe best you can do is create a set of
RewriteRules that will do header redirects to the target domain. However, in that scenario, the target domain/IP will always be visible to the end user.