My first attempt at retrieving data from XML for a maps application has failed. Here is a piece of the XML Feed.
Haven't tested your specific case, but i remember running into something similar when using SimpleXML, you might want to use (string) to cast it out of the object
array('output'=> (string)$start[0])
Or rather just leave out $start = array($start) and just do
array('output'=> (string)$start)
On reading the SimpleXML XPath documentation (http://www.php.net/manual/en/simplexmlelement.xpath.php) again i think your problem might be this:
Returns an array of SimpleXMLElement objects or FALSE in case of an error.
So the XPath returns an array, then you wrap that in an array and take the first element of that array, so all you end up with is the original array - remove the array wrap and you should be fine