How to get the value of an attribute from XML file in PHP?

Sorry if this seems like an easy question, but I've started pulling hair out on this...

I have a XML file which looks like this...

<VAR VarNum="90">
  <option>1</option>
</VAR>

I'm trying to get the VarNum.

So far I've been successful using the follow code to get the other information:

$xml=simplexml_load_file($file);
$option=$xml->option;

I just can't get VarNum (the attribute value I think?)

Thanks!