Firebug? Do you mean the plug-n for Firefox? If so, you need to have a very good grounding in HTML and CSS before even considering using it.

PHP is a server side technology and CSS is a style sheet to format your web pages, there is no comparison between the two. It's impossible for a browser (with or without the Firebug plug-in) to tell what has been done by PHP as it is done on the server behind closed doors before the page is sent to the browser.

Firebug only deals with the HTML and CSS that is sent to the browser, PHP doesn't come into it.

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Lawrence