- simplexml_load_string: documentation ( source)
<?php
$x = <<<XML
<data type="current observations">
<time-layout time-coordinate="local">
<start-valid-time period-name="current">2013-05-27T13:53:00-04:00</start-valid-time>
</time-layout>
</data>
XML;
$weather = simplexml_load_string($x);
$data = $weather->xpath('/data[@type="current observations"]')[0];
$time_layout = $data->xpath('time-layout[@time-coordinate="local"]')[0];
$start_valid_time = $time_layout->xpath('start-valid-time[@period-name="current"]')[0];
echo (string)$start_valid_time;