source: https://www.securityfocus.com/bid/3107/info

PHP-Nuke is a website creation/maintenance tool written in PHP3.

If a malicious user may subtitute arbitrary values for image form elements in the PHP-Nuke User Registration Form by saving the webpage locally(as 'user.php.html') and altering the information. The new values may be set in such a way as to disclose arbitrary web-readable files to the attacker.

http://www.target.com/../../../dir_on_server/anyfile.ext

It will load the file when the user info of the attacker is viewed.