We have a PHP Project under SVN control(VisualSVN).
Somehow, i need to update a file in the repo with the current revision number. So that when the main site gets updated/checkout we can show the svn build number (this part is not the issue).
is it possible with a pre/post-commit-hook to do this?
Sounds like you need a build solution, rather than a pre-commit hook. Using a build system like Apache Ant, you could grab the latest revision number from the repository when preparing your solution for the live environment.
Edit: To actually answer your question "is it possible with a pre/post-commit-hook to do this?" yes it is technically possible but as already suggested, it should be avoided.