Description:
------------
XPath (and possibly libxml2) has a bug when selecting attributes with a namespace. We must use the namespace-uri() function to solve this problem whereas using a registered prefix does not work.

A bug has been reported to Gnome in 2008: <https://bugzilla.gnome.org/show_bug.cgi?id=562557> but it's still opened and not resolved.

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

$xml = '<?xml version="1.0" encoding="utf-8"?>' . "\n\n" .
       '<foo xmlns="http://ns1" xmlns:x="http://ns2">' . "\n" .
       '  <bar a="b" x:c="d" e="f" g="h" />' . "\n" .
       '</foo>';

$sxe = simplexml_load_string($xml);

$sxe->registerXPathNamespace('',  'http://ns1');
$sxe->registerXPathNamespace('x', 'http://ns2');

echo $xml . "\n\n" .
     '✘ //@*                                 found(' .
     count($sxe->xpath('//@*')) . ')' . "\n" .
     '✘ //@x:* (x registered as http://ns2)  found(' .
     count($sxe->xpath('//@x:*')) . ')' . "\n" .
     '✔ //@*[namespace-uri() = "http://ns2"] found(' .
     count($sxe->xpath('//@*[namespace-uri() = "http://ns2"]')) . ')' . "\n";

Expected result:
----------------
<?xml version="1.0" encoding="utf-8"?>

<foo xmlns="http://ns1" xmlns:x="http://ns2">
  <bar a="b" x:c="d" e="f" g="h" />
</foo>

✘ //@*                                 found(4)
✘ //@x:* (x registered as http://ns2)  found(1)
✔ //@*[namespace-uri() = "http://ns2"] found(1)

Actual result:
--------------
<?xml version="1.0" encoding="utf-8"?>

<foo xmlns="http://ns1" xmlns:x="http://ns2">
  <bar a="b" x:c="d" e="f" g="h" />
</foo>

✘ //@*                                 found(4)
✘ //@x:* (x registered as http://ns2)  found(4)
✔ //@*[namespace-uri() = "http://ns2"] found(1)