Yep, pretty much the same as above:
function sortByKBPSASC($a, $b)
$iResult = 0;
if (is_object($a) && is_object($b) && ($a instanceof SimpleXMLElement) && ($b instanceof SimpleXMLElement))
$aAttr = $a->attributes();
$bAttr = $b->attributes();
if (isset($aAttr['kbps'], $bAttr['kbps']))
$iResult = $aAttr['kbps'] - $bAttr['kbps'];
//. . .
Prior to the foreach. Works okay in my head, not sure how it'll turn out though :P
With these types of changes though, you may want to consider making more use of the xpath functionality. The downside of that is that the libxml libraries will dictate what will work with it, so you may not be able to use xpath 2.0 functionality if you only have a 1.0 library for example.
Hindsight is 20/20, you could probably typehint this method instead of using the object and instanceof checks.