Description:
------------
SimpleXML misses a XPath function which is processing-instruction(). This last one enables to reach/select PIs along the XML document.

Test script:
---------------
<?php

$xml = '<?xml version="1.0" encoding="utf-8"?>' . "\n\n" .
       '<foo>' . "\n" .
       '  <bar>Gordon</bar>' . "\n" .
       '  <bar><![CDATA[Gordon]]></bar>' . "\n" .
       '  <bar><?baz href="ftw" ?></bar>' . "\n" .
       '</foo>';

$sxe = simplexml_load_string($xml);

var_dump(
    $sxe->xpath('//bar')
);

var_dump(
    $sxe->xpath('//processing-instruction(\'baz\')')
);

Expected result:
----------------
//bar

array(3) {
  [0]=>
  object(SimpleXMLElement)#2 (1) {
    [0]=>
    string(6) "Gordon"
  }
  [1]=>
  object(SimpleXMLElement)#3 (0) {
  }
  [2]=>
  object(SimpleXMLElement)#4 (1) {
    ["baz"]=>
    object(SimpleXMLElement)#5 (0) {
    }
  }
}


//processing-instruction('baz')
array(1) {
  [2]=>
  object(SimpleXMLElement)#6 (1) {
    …
  }
}

Actual result:
--------------
//bar

array(3) {
  [0]=>
  object(SimpleXMLElement)#2 (1) {
    [0]=>
    string(6) "Gordon"
  }
  [1]=>
  object(SimpleXMLElement)#3 (0) {
  }
  [2]=>
  object(SimpleXMLElement)#4 (1) {
    ["baz"]=>
    object(SimpleXMLElement)#5 (0) {
    }
  }
}


//processing-instruction('baz')
array(0) {
}