PHP und XML Float und Double

$preis =  20.55;


 $test_array = array (
            $preis => 'AmmountWithTax',  
);


 $xml = new SimpleXMLElement('<?xml version="1.0" encoding="UTF-8" ' . 'standalone="yes"?><root/>');  
        array_walk_recursive($test_array, array ($xml, 'addChild'));  


        $xmlAusgabe = $xml->asXML();
        $fp = fopen("files/test/Export.xml", "a+");  
        fwrite ($fp, $xmlAusgabe);

<?xml version="1.0" encoding="UTF-8" standalone="yes"?> 
<root>
<AmmountWithTax>20</AmmountWithTax>
</root>