Getting values from Wunderground API with SimpleXML in PHP

Have been experimenting with a number of weather xml api's, have followed same process/logic for each and worked fine with a number of them but Wunderground API has me stumped.

$XML_WUNDER = simplexml_load_string('mylocalcopyofapiresponse.xml', null, LIBXML_NOCDATA)
print_r($xml_wunder);

Results in:

SimpleXMLElement Object
(
[version] => 0.1
[termsofService] => http://ift.tt/1c1wgoz
[features] => SimpleXMLElement Object
    (
        [feature] => forecast10day




    )

[forecast] => SimpleXMLElement Object
    (
        [txt_forecast] => SimpleXMLElement Object
            (
                [date] => 11:20 PM NZST
                [forecastdays] => SimpleXMLElement Object
                    (
                        [forecastday] => Array
                            (
                                [0] => SimpleXMLElement Object
                                    (
                                        [period] => 0
                                        [icon] => sleet
                                        [icon_url] => http://ift.tt/XfFTAM
                                        [title] => Thursday
                                        [fcttext] => A mix of wintry precipitation in the morning. Then periods of rain expected in the afternoon. High 37F. Winds N at 5 to 10 mph. Chance of precip 100%. Snow and ice accumulating 1 to 3 inches.
                                        [fcttext_metric] => A mix of wintry precipitation in the morning. Then periods of rain expected in the afternoon. High 3C. Winds N at 10 to 15 km/h. Chance of precip 100%. 3-7cm of snow and ice expected.
                                        [pop] => 100
                                    )

                                [1] => SimpleXMLElement Object
                                    (
                                        [period] => 1

.....trimmed for brevity

The original response is as shown in the apigee console at the bottom of this page http://ift.tt/ULw8sn

I would expect

$xml_wunder->forecast->txt_forecast->forecastdays->forecastday[0]->fcttext_metric

To return:

A mix of wintry precipitation...........cip 100%. 3-7cm of snow and ice expected. 

But get no response at all. This is on a page (xampp localhost) with two other weather xml api responses from different sources, all use same cache function and are identical in mark up, the others work as expected.

I suspect something to do with how the array is formatted as is not consistent with other api responses, leading to an error in my formatting. As such there's potentially a suitable answer out there, I'm just not aware of the terms to search for in order to get there.

Any pointers gratefully received.