<?php

$my_xml_data = 
'<?xml version="1.0" encoding="UTF-8"?>
<student>
    <?sort alpha-ascending?>
    <?textinfo whitespace is allowed ?>
    <first-name>George</first-name>
    <phone.mobile>(011) 123-4567</phone.mobile>
</student>';

$xml = simplexml_load_string($my_xml_data);

$doc   = dom_import_simplexml($xml)->ownerDocument;
$xpath = new DOMXPath($doc);

# prints "/* processing instructions */ ", the value of the first PI: textinfo
echo $xpath->evaluate('string(//processing-instruction("textinfo")[1])');

whitespace is allowed