- simplexml_load_string: documentation ( source)
<?php
$str=<<<XML
<root>
<A>
<someid>5</someid>
</A>
<B>
<someid>15</someid>
</B>
</root>
XML;
$xml=simplexml_load_string($str);
foreach($xml->xpath('//someid[text()="5"]') as $node)
{
echo $node;
}