php XML转json

$x=new xml();
$x->xml2json();
class xml
{
    function xml2json()
    {
        $xml_object=NULL;
        if (file_exists('test.xml')) 
        {
            $xml_object = simplexml_load_file('test.xml');
        } 
        else 
        {
            exit('Failed to open test.xml.');
        }        
        $xml_json=json_encode($xml_object);
        echo $xml_json;
    }
}