PHP Shell is a shell wrapped in a PHP script. It’s a tool you can use to execute arbitrary shell-commands or browse the filesystem on your remote webserver. This replaces, to a degree, a normal telnet connection, and to a lesser degree a SSH connection.
You use it for administration and maintenance of your website, which is often much easier to do if you can work directly on the server. For example, you could use PHP Shell to unpack and move big files around. All the normal command line programs like ps, free, du, df, etc… can be used.http://phpshell.sourceforge.net/