Description:
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If you add a new namespace to the DOM Document then the XSLT gives an error "undefined namespace". And if you save the DOM and load again it works.

Reproduce code:
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$oDOM = new DOMDocument();
$oDOM->load('random_file.xsl'); //Must be a valid XSL file with a <xsl:stylesheet> tag
$oDOM2 = new DOMDocument();
$oDOM2->load('random_file.xml'); //Must be a valid XML file

$oElement = $oDOM->getElementsbyTagNameNS('*', 'stylesheet')->item(0);
$oElement->setAttribute('xmlns:exsl', 'http://exslt.org/common');

$oXSLTransform = new XSLTProcessor();
$oXSLTransform->importStyleSheet($oDOM);
$oOutput = $oXSLTransform->transformToDoc($oDOM);

Expected result:
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A transformed DOC document (without errors)

Actual result:
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Warning: XSLTProcessor::importStylesheet() [xsltprocessor.importstylesheet]: compilation error: file file.xsl line 3 element stylesheet in file.php on line 13
Warning: XSLTProcessor::importStylesheet() [xsltprocessor.importstylesheet]: xsl:extension-element-prefix : undefined namespace exsl in file.php on line 13