Environment::extPath() 

\nn\t3::Environment()->extPath($extName = ''); 

Get the absolute path to an extension e.g. /var/www/website/ext/nnsite/

\nn\t3::Environment()->extPath('extname');

| @return string

public function extPath ( $extName = '' ) {
	return ExtensionManagementUtility::extPath( $extName );
}