PHP & HTML Purifier error: Undefined variable: dirty_html-Collection of common programming errors

I’m trying to install HTML Purifier http://htmlpurifier.org/ but I get the following error Undefined variable: dirty_html. I was wondering how can I fix this problem?

Here is the PHP code.

 require_once '../../htmlpurifier/library/HTMLPurifier.auto.php';

 $config = HTMLPurifier_Config::createDefault();
 $config->set('Core.Encoding', 'UTF-8'); // replace with your encoding
 $config->set('HTML.Doctype', 'XHTML 1.0 Strict'); // replace with your doctype
 $purifier = new HTMLPurifier($config);

 $clean_html = $purifier->purify($dirty_html);

Here is my html form.

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