<?php
$xml = <<<XML
<?xml version="1.0" standalone="yes"?>
<people xmlns:p="http://example.org/ns" xmlns:t="http://example.org/test">
<p:person id="1">John Doe</p:person>
<p:person id="2">Susie Q. Public</p:person>
</people>
XML;
$sxe = new SimpleXMLElement($xml);
$namespaces = $sxe->getNamespaces(true);
var_dump($namespaces);
?>
Результат выполнения данного примера:
array(1) {
["p"]=>
string(21) "http://example.org/ns"
}