- 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);
$time=$weather->xpath("//data[@type='current observations']/time-layout/start-valid-time");
echo $time[0];