PHP SimpleXML get document node

I need to extract info from an xml document's outermost node (the document itself). using the xml below:

<?xml version="1.0" encoding="UTF-8" ?>
<revue date="2015" issue="12345">
  <innernode>Oi</innernode>
</revue>

i would like to extract 2015 and 12345. i fail if i attempt to read the node revue using a foreach loop

foreach ($xml->revue as $revueIn) {
    foreach ($revueIn->attributes() as $z => $y) { 

to see if i have typos or something, i inverted innernode and revue

<?xml version="1.0" encoding="UTF-8" ?>
<innernode>
  <revue date="2015" issue="12345">Oi</revue>
</innernode>

and above code worked (found the revue node and read its attributes correctly)

therefore, it seems to me that the outermost node is treated/perceived differently by SimpleXML.

does anyone know how to access the outermost node? tks in advance 4 your time.