It could be a variety of set-ups.

For example forum.blahblah.com/content and http://www.blahblah.co.uk can be on completely different machines at opposite ends of the Earth.

Just because the domain names are similar does not have to mean that the underlying implementations are joined.

Try doing a DNS lookup on the domain names and use some web tools to investigate the site’s IP addresses – this might shed light on it.

See: http://www.dnsstuff.com/

Personally speaking, If my main site was .NET and I was adding a 3rd party PHP forum – I’d stick it on a different machine.

Update in response to the “Why Are You Asking” comment discussion, below

“basically trying to figure out how to make asp.net website and php based forum work together”

I wouldn’t get too hung up on making them work together ‘physically on a machine’.
I’ve spend hours trying to untangle, separate and modularize my code and so I hate to see you struggling to merge yours together. ;o)

Keeping them as separate systems and defining the HTTP interface between them is probably simpler.

If you want ‘one login’ for both components, then that is possible to keep them separate AND share identity using things like OpenID
– http://openid.net/developers

Quote: “OpenID is a decentralized authentication protocol”

This video on REST is a brilliant intro to the (often overlooked) power of HTTP
– http://www.youtube.com/watch?v=YCcAE2SCQ6k

Quote: “REST is the architecture of the web as it works today…Shouldn’t you be working with the architecture instead of against it”

And perhaps look up Eric Evan’s chapter on ‘Separate Ways’ in his DDD book.

• http://books.google.co.uk/books?id=7dlaMs0SECsC&lpg=PP1&dq=domain%20driven%20design&pg=PA335#v=onepage&q&f=false

Quote: “Integration is always expensive. Sometimes the benefit is small”

All these are non-language specific solutions.

Also you might find this talk about how the BBC integrate their website interesting.
– http://www.infoq.com/presentations/web-20-bbc-scaling

Gist: They keep everything separated as much as possible, rarely sending messages between machines in anything other than simple HTTP requests. i.e They avoid remote-SQL queries because they are harder to track and log.

** Evil Character Encoding Issues **

When you start writing your own ‘web readers’ its worth being aware of character encoding issues too:

Read this article by, Mr Stackoverflow himself, Joel Spolsky:

The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!) – http://www.joelonsoftware.com/articles/Unicode.html

** What the users see**

OK – that was about getting the two systems working together over the web. But, if you want one interface, one domain name to act as the ‘gateway’, then you could choose to make either the ASP server, or the PHP Lamp Server the ‘frontend’. This is where it can get complex.
I only know about Apache, not MS..so

I run a number of sites that have different IP addresses, hosted on different machines – but the code for all of them sits on one machine. Each of the Satellite Sites simply proxies HTTP requests to my ‘Origin Server‘ and forwards the reposnses back to the user.

This is done using the apache’s proxy features.
– http://httpd.apache.org/docs/2.0/mod/mod_proxy.html

Quote:

A typical usage of a reverse proxy is
to provide Internet users access to a
server that is behind a firewall.
Reverse proxies can also be used to
balance load among several back-end
servers, or to provide caching for a
slower back-end server. In addition,
reverse proxies can be used simply to
bring several servers into the same
URL space.

Be prepared for headaches – but with dedication its possible to have two servers one domain name.

See: mod rewrite:

• http://httpd.apache.org/docs/current/mod/mod_rewrite.html

Quote:

‘proxy|P’ (force proxy)

This flag forces the substitution part to be internally
sent as a proxy request and
immediately (rewrite processing stops
here) put through the proxy module.
You must make sure that the
substitution string is a valid URI
(typically starting with
http://hostname) which can be handled
by the Apache proxy module. If not,
you will get an error from the proxy
module. Use this flag to achieve a
more powerful implementation of the
ProxyPass directive, to map remote
content into the namespace of the
local server.

    Note: mod_proxy must be enabled in order to use this flag.

I, like you, also racked my brains trying to figure this stuff out. The trick is to know the right words.

The following phrases probably more likely to hold the key to your answers

• network architecture
• REST
• gateway
• proxy
• syndication
• XML/RPC
• SOAP
• API

…than these phrases:

• make asp work with php
• two different web sites
• etc