Getting all namespaces when using simplexml_load_string()

I'm getting the following response back from the Amazon MWS API:

<?xml version="1.0"?>
<GetMatchingProductForIdResponse xmlns="http://ift.tt/1iBZrs1">
<GetMatchingProductForIdResult Id="B00002N97S" IdType="ASIN" status="Success">
    <Products xmlns="http://ift.tt/1iBZrs1" xmlns:ns2="http://ift.tt/1iBZq7r">
    <Product>
        <Identifiers>
            <MarketplaceASIN>
                <MarketplaceId>ATVPDKIKX0DER</MarketplaceId>
                <ASIN>B00002N97S</ASIN>
            </MarketplaceASIN>
        </Identifiers>
        <AttributeSets>
            <ns2:ItemAttributes xml:lang="en-US">
                <ns2:Binding>Tools &amp; Home Improvement</ns2:Binding>
                <ns2:Brand>Moen</ns2:Brand>
                <ns2:Color>White</ns2:Color>
                <ns2:Department>Fixtures</ns2:Department>
                <ns2:Feature>Fits all Moen 2 handle faucets except Roman tubs</ns2:Feature>
                <ns2:Feature>Solid brass construction</ns2:Feature>
                <ns2:Feature>Package contains only 1 cartridge</ns2:Feature>
                <ns2:Feature>Replaces both hot and cold cartridges</ns2:Feature>
                <ns2:Feature>Also used in 2 and 3 handle tub showers</ns2:Feature>
                <ns2:Feature>Installation instructions included</ns2:Feature>
                <ns2:ItemDimensions>
                    <ns2:Height Units="inches">6.50</ns2:Height>
                    <ns2:Length Units="inches">1.30</ns2:Length>
                    <ns2:Width Units="inches">2.10</ns2:Width>
                </ns2:ItemDimensions>
                <ns2:Label>MOEN</ns2:Label>
                <ns2:ListPrice>
                    <ns2:Amount>13.85</ns2:Amount>
                    <ns2:CurrencyCode>USD</ns2:CurrencyCode>
                </ns2:ListPrice>
                <ns2:Manufacturer>MOEN</ns2:Manufacturer>
                <ns2:MaterialType>Brass</ns2:MaterialType>
                <ns2:Model>1224</ns2:Model>
                <ns2:NumberOfItems>1</ns2:NumberOfItems>
                <ns2:PackageDimensions>
                    <ns2:Height Units="inches">1.30</ns2:Height>
                    <ns2:Length Units="inches">6.50</ns2:Length>
                    <ns2:Width Units="inches">2.30</ns2:Width>
                    <ns2:Weight Units="pounds">0.05</ns2:Weight>
                </ns2:PackageDimensions>
                <ns2:PackageQuantity>1</ns2:PackageQuantity>
                <ns2:PartNumber>1224</ns2:PartNumber>
                <ns2:ProductGroup>Home Improvement</ns2:ProductGroup>
                <ns2:ProductTypeName>PLUMBING_FIXTURE</ns2:ProductTypeName>
                <ns2:Publisher>MOEN</ns2:Publisher>
                <ns2:SmallImage>
                    <ns2:URL>http://ift.tt/1M33Zmz;
                    <ns2:Height Units="pixels">75</ns2:Height>
                    <ns2:Width Units="pixels">75</ns2:Width>
                </ns2:SmallImage>
                <ns2:Studio>MOEN</ns2:Studio>
                <ns2:Title>MOEN 1224 Replacement Faucet Cartridge Kit Fits Most Double Handle Moen Faucets and Tub/Showers</ns2:Title>
            </ns2:ItemAttributes>
        </AttributeSets>
        <Relationships>
            <VariationParent>
                <Identifiers>
                    <MarketplaceASIN>
                        <MarketplaceId>ATVPDKIKX0DER</MarketplaceId>
                        <ASIN>B00B4LDOIY</ASIN>
                    </MarketplaceASIN>
                </Identifiers>
            </VariationParent>
        </Relationships>
        <SalesRankings>
            <SalesRank>
                <ProductCategoryId>home_improvement_display_on_website</ProductCategoryId>
                <Rank>1055</Rank>
            </SalesRank>
            <SalesRank>
                <ProductCategoryId>542644011</ProductCategoryId>
                <Rank>4</Rank>
            </SalesRank>
        </SalesRankings>
    </Product>
</Products>
</GetMatchingProductForIdResult>
<ResponseMetadata>
    <RequestId>02xnfr50-fdfa-5dg4-9ffc-xb6bx651fcb</RequestId>
</ResponseMetadata>
</GetMatchingProductForIdResponse>

Whenever I process this using:

$xml = simplexml_load_string($xml_string, 'SimpleXMLElement', LIBXML_NOCDATA);

...I find that the entire AttributeSets tree is empty in the $xml variable.

How do I load the string and get all the namespaces in one result?

PHP 5.6.