It is mentioned in a few places on the web, but it wasn't mentioned here. You need to use this function to set up a prefix for the default namespace of a document.
For instance, if you are trying to parse a Microsoft Spreadsheet XML file, which has the default namespace of "urn:schemas-microsoft-com:office:spreadsheet":
$doc = DOMDocument::load("my_spreadsheet.xml);
$xpath = new DOMXPath($doc);
$xpath->registerNamespace("m",
"urn:schemas-microsoft-com:office:spreadsheet");
$query = '/m:Workbook/m:Worksheet[1]/m:Table';
$result = $xpath->query($query, $doc);
You can use anything in place of the 'm', but you have to specify something! Just asking for "/Workbook/Worksheet/Table" doesn't work.