In this new post on the PHP Security Blog, Stefan Esser brings up an interesting issue when dealing with web applications - being able to bind a script/application to a different port on the local machine.
Unfortunately the ability to bind yourself to a port and receive connections is a threat to webapplications installed on different virtual hosts on the same IP, even if other security measures in place, like tight filesystem permissions or executing PHP script with the permission of the owner.
He gives an example in PHP of how this can happen and one of the scary side effects of having it in the same domain - the cookie information is passed in. It's a simple concept that could have very bad consequences in the wrong hands.
Additionally, you don't even have to create it in PHP either. As cyberlot mentions, it could be created in any language that has socket functions. Be sure to check out the comments for more thoughts and comments on the post.