PHP SimpleXml can not access a node value using : character

I do load an XML string using :

$xml = simplexml_load_string($data['MyXml']['xml']);

And could get the value from all the nodes using this sintax:

$xml->MyNode->AnotherNode->MyFinalNode[0];

But there is a node that uses : as a separator and i could not get the value :

$xml->MyNode->AnotherNode->abc:MyFinalNode[0];

How can i access the value from this node using PHP simplexml functions ?

For example, how can i access the D node into :

<my:A>  
    <my:B>
        <my:C>  
            <my:D>