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;
    }
}