With an introduction of a new Firefox extension in PHP, Terry Chay comments in his blog about the nitty-gritty that he had experienced working on it. He first explains the purpose that this extension allows PHP scripts to send debugging or profiling information to Firefox’s Firebug without having to write the page itself. He says the standard way of debugging in PHP involves echoing code to the screen. The fact that PHP is a scripting language makes this an way of debugging on the Web. But this may lead to some breaks in your site. This is especially relevant when you are debugging a well formed XML or JSON response, he says.

Source: Debugging PHP with Firebug