In an effort to just about everything he can with PHP Mark Karpeles has posted an article about how he created a SSH server in PHP with most (not quite all) of the functionality of some of its counterparts.

You probably know SSH at least by its name. It's a of secure telnet replacement which also allows many other things such as port forwarding, remote file management (with sftp) and more. With PHP I could write a fully working SSH server in only 3 days. [...] My goal when writing this was to provide a replacement for the FTP protocol for the customers of my hosting service.

He lists the extensions used to create the extension - OpenSSL, mcrypt, hash and (the most important) GMP as well as a link to the source if you'd like to try it out yourself.