FirePHP

FirePHP enables you to print to your Firebug Console using a simple PHP function call.

What makes FirePHP different?

All data is sent via a set of X-FirePHP-Data response
headers. This means that the debugging data will not interfere with the
content on your page. Thus FirePHP is ideally suited for AJAX
development where clean JSON or XML responses are required.

Requirements

PHP:

5

Firefox:

2, 3

Firebug:

1.05, 1.1, 1.2

http://firephp.org/