Apparatus by Mathew Tinsley is a web-based PHP read-evaluate-print interpreter that allows you to write and execute PHP code directly from your wordpress admin area.
The source code is displayed back to you with syntax highlighting along with any output. Errors, warnings, and notices are also displayed. Apparatus is great for developing code to be added to wordpress posts. WordPress developers may also find it useful for testing code, updating options, and debugging, all without needing to leave your wordpress admin area.
There is a Stand-Alone and a WordPress plugin version, more info and download at: http://tinsology.net/scripts/apparatus/