Description:
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Take a look on a bug: http://bugs.php.net/bug.php?id=43289
which was incorrectly marked as "bogus"

I'm talking exactly what i'm talking. Please double check what the function document('') should do. 

take attention to the line:

$xpath->query('//xsl:stylesheet')->item(0)->appendChild($xpath->query('/
/xsl:template')->item(0)->cloneNode(true));

which clones xsl:template node, so we have two <xsl:template match="/"> nodes in result template, and you're right only one will match, BUT this template parses ITSELF:

<xsl:for-each select="document('')//xsl:*">

You can comment line:
//$xml->load('/tmp/template.xsl');

to have a little understanding what i'm talking about.

So, it should find TWO <xsl:template match="/"/> nodes, and output should be:

xsl:stylesheet
xsl:output
xsl:template
xsl:for-each
xsl:value-of
xsl:text
xsl:template
xsl:for-each
xsl:value-of
xsl:text

And i surprize you: when the template DOM initialized by DOMDocument::loadXML() instead of DOMDocument::load() it gave the expected output.