Xpath actually knows which element is root regardless of element on which you execute it. Example:

<?php

$string = <<<XML
<a>
 <b>
  <c>text</c>
  <c>stuff</c>
 </b>
 <b>
  <c>code</c>
 </b>
</a>
XML;

header('content-type: text/plain');

$xml = new SimpleXMLElement($string);

//relative to root
$b0=$xml->b[0]->xpath('//c');
while(list( , $node) = each($b0)) {
    echo 'b[0]: //c: ',$node,"\n";
}

$b1=$xml->b[1]->xpath('//c');
while(list( , $node) = each($b1)) {
    echo 'b[1]: //c: ',$node,"\n";
}

echo "\n";

//relative to current element
$b0=$xml->b[0]->xpath('.//c');
while(list( , $node) = each($b0)) {
    echo 'b[0]: .//c: ',$node,"\n";
}

$b1=$xml->b[1]->xpath('.//c');
while(list( , $node) = each($b1)) {
    echo 'b[1]: .//c: ',$node,"\n";
}

?>

Will return:
b[0]: //c: text
b[0]: //c: stuff
b[0]: //c: code
b[1]: //c: text
b[1]: //c: stuff
b[1]: //c: code

b[0]: .//c: text
b[0]: .//c: stuff
b[1]: .//c: code