PHP Document Root path is incorrect?

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http://serverfault.com – I am using nginx and apache server, only one apache server is running with usr/local/lib/apache (cpanel default apache server) and when i call the document root folder like this it $_SERVER['DOCUMENT_ROOT'] gives me the /usr/local/apache/htdocs while the correct location should have been the /home/xxx/public_html/ any idea on how to fix this ? is this nginx problem or php settings problem or the apache configuration problem ? (HowTos)