In a new post on his blog, Guy Harpaz answers a few questions people have been having about the debugger protocol that the PHP IDE project uses.

Debugging a PHP application or a PHP web server requires connectivity between an IDE and a Debugger engine (a PHP module which is installed on the web server). The debug protocol defines this connection. [...] When the Eclipse Foundation approved the PHP IDE project, Zend Studio's debug protocol was opened source and was chosen to be the debug protocol of the PHP IDE project.

He goes through why they made the choice, touches a bit on the security aspect of the two debugging protocols he mentioned (DBGp and Zend Studio's) as well as their common methods for output.