Thank you very much for your help, I finally manage to autenticate and retrieve files from my rpc server.. Here is my syntax in case anyone needs it
// USER LOGIN $authenticate = xmlrpc($settings['server'], Array('user.login' => array($settings['user'] , $settings['password']),));
// CREATE HEADERS $options['headers']['Cookie'] = $autenticate['session_name'] . '=' . $autenticate['sessid'];
//RETRIEVE FILE $file= xmlrpc($settings['server'], Array('file.retrieve' => Array(Array("fid"=>$fid),Array("file_contents"=> 'true'),Array("image_styles"=>'true') )),$options);