Description:
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Because XPath cannot select nodes from the default namespace (i.e. xmlns="foo") an alias must be added. This is possible in the DOM extension using the DomXpath::registerNamespace() method. Because SimpleXML does not have this, its XPath implementation is quite crippled.

The only way around this is the following:

$sxml = simplexml_load_*($xml)

$sxml['xmlns:foo'] = 'http://foo'; 

$sxml = simplexml_load_string($sxml->asXML());

Which is quite stupid really.

- Davey