Description:
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DOMDocument should extend DOMNode (http://www.w3.org/TR/DOM-Level-3-Core/core.html#i-Document), and should be able to find any xmlns defined in the top level element.

Currently, DOMDocument->lookupPrefix() and DOMDocument->lookupNamespaceURI() will never return any values other than null; or warn developers they are using the wrong method.

Additionally, DOMDocument->lookupPrefix & friends should be able to recognise xmlns defined in the root node of a document.

Reproduce code:
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<?php
$xml = '<?xml version="1.0" encoding="UTF-8"?>
<office:document-content office:class="text" office:version="1.0" xmlns:chart="http://openoffice.org/2000/chart" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:dom="http://www.w3.org/2001/xml-events" xmlns:dr3d="http://openoffice.org/2000/dr3d" xmlns:draw="http://openoffice.org/2000/drawing" xmlns:fo="http://www.w3.org/1999/XSL/Format" xmlns:form="http://openoffice.org/2000/form" xmlns:math="http://www.w3.org/1998/Math/MathML" xmlns:meta="http://openoffice.org/2000/meta" xmlns:number="http://openoffice.org/2000/datastyle" xmlns:office="http://openoffice.org/2000/office" xmlns:ooo="http://openoffice.org/2004/office" xmlns:oooc="http://openoffice.org/2004/calc" xmlns:ooow="http://openoffice.org/2004/writer" xmlns:script="http://openoffice.org/2000/script" xmlns:style="http://openoffice.org/2000/style" xmlns:svg="http://www.w3.org/2000/svg" xmlns:table="http://openoffice.org/2000/table" xmlns:text="http://openoffice.org/2000/text" xmlns:xforms="http://www.w3.org/2002/xforms" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <office:body>
        <table:table table:name="Table13" table:style-name="Table13">
            <table:table-column table:style-name="Table13.A" />
            <table:table-row table:style-name="Table13.1">
                <table:table-cell table:style-name="Table13.A1" table:value-type="string">
                    <text:p text:style-name="P84">Important Notes and Qualifications</text:p>
                </table:table-cell>
            </table:table-row>
        </table:table>
        <text:p text:style-name="P88">&lt; &lt; func_image(photograph) &gt; &gt;</text:p>
    </office:body>
</office:document-content>';

$document = new DOMDocument('1.0', 'UTF-8');
$document->loadXML($xml);

foreach ($document->getElementsByTagNameNS('http://openoffice.org/2000/table', 'table') as $element) {
    echo 'local name: ', $element->localName, ', prefix: ', $element->prefix, ' xmlns (element):', $element->lookupNamespaceURI($element->prefix), ' xmlns (document):', $document->lookupNamespaceURI($element->prefix), "\n";
}
print "\n\n";
foreach ($document->getElementsByTagName('*') as $element) {
    echo 'local name: ', $element->localName, ', prefix: ', $element->prefix, ' xmlns (element):', $element->lookupNamespaceURI($element->prefix), ' xmlns (document):', $document->lookupNamespaceURI($element->prefix), "\n";
}
print "\n\n";
foreach ($document->getElementsByTagNameNS('*', '*') as $element) {
    echo 'local name: ', $element->localName, ', prefix: ', $element->prefix, ' xmlns (element):', $element->lookupNamespaceURI($element->prefix), ' xmlns (document):', $document->lookupNamespaceURI($element->prefix), "\n";
}

print "\n\n";
foreach ($document->getElementsByTagNameNS('http://openoffice.org/2000/table', '*') as $element) {
    echo 'local name: ', $element->localName, ', prefix: ', $element->prefix, ' xmlns (element):', $element->lookupNamespaceURI($element->prefix), ' xmlns (document):', $document->lookupNamespaceURI($element->prefix), "\n";
}

print "\n\n";
foreach ($document->getElementsByTagNameNS('http://openoffice.org/2000/table', '*') as $element) {
    echo 'local name: ', $element->localName, ', prefix: ', $element->prefix, ' xmlns (element):', $element->lookupNamespaceURI($element->prefix), ' xmlns (document):', $document->lookupNamespaceURI($element->prefix), "\n";
}

print "\n\n";
foreach ($document->getElementsByTagNameNS('http://openoffice.org/2000/table', 'table') as $element) {
    echo 'local name: ', $element->localName, ', prefix: ', $element->prefix, ' xmlns (element):', $element->lookupNamespaceURI($element->prefix), ' xmlns (document):', $document->lookupNamespaceURI($element->prefix), "\n";
}

Expected result:
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$document->lookupNamespaceURI() and $element->lookupNamespaceURI() should return identical results.

Actual result:
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local name: table, prefix: table xmlns (element):http://openoffice.org/2000/table xmlns (document):