This is because the dot .is a string concatenator in php. In your case, it tries to concatenate $xmlobj->News->News(which does not exist and is therefore empty) with a constant Env(which also does not exist and is treated as a string). You will receive a notification about this using the appropriate error_level)
$tmp = 'News.Env';
$xmlobj->News->$tmp;
or shorter
$xmlobj->News->{'News.Env'};
Update: if you use SimpleXML(and according to the syntax that you do this), it $xmlobj"starts" with the element News- (root-).
$xmlobj->{'News.Env'};