Your question wasn't really focused enough to give a single direct correct answer.
We built a community website which is
based on ColdFusion. Is it altogether
possible to add features such as
blogs, and other community features to
our website in PHP?
I mean to combine both PHP and
ColdFusion together.
When you say you want to "combine" them, what exactly do you mean? They can both co-exist on a web server and both have access to the same database; so if you want both PHP and ColdFusion applications to have access to the same data, that is not a problem at all. There would be some concerns about locking and race conditions, but from an "is this possible?" standpoint, the answer is undoubtedly YES.
Sharing session, client, or other special scopes/variables from CF to PHP should be possible with some clever programming, but would be tricky. It would depend entirely on your needs and your implementation.
If you're looking for applications to add to your community, I would recommend that you try to find something in CFML that suits your needs before you go the route of attempting to mix CFML and PHP. There are tons of free open source applications including blogs and wikis written in CFML available on RIAForge, and other websites. Canvas Wiki, and Codex Wiki are solutions that focus strictly on Wiki functionality, and there are others that include Wiki functionality as part of a larger package. Searching for "blog" in the ColdFusion category on RIAForge shows no less than 6 blogging applications like BlogCFC and others, and that doesn't even include my personal favorite, Mango Blog.