如何能知道XML对象中有几个一层的元素simplexml_load_file()

object(SimpleXMLElement)#24 (1)
{
["page"]=> array(2)
{
[0]=> object(SimpleXMLElement)#25 (2)
{
["@attributes"]=> array(1)
{
["id"]=> string(1) "0"
}
["background"]=> object(SimpleXMLElement)#38 (1)
{
["@attributes"]=> array(1)
{
["color"]=> string(11) "255,255,255"
}
}
}
[1]=> object(SimpleXMLElement)#26 (1)
{
["@attributes"]=> array(1)
{
["preview"]=> string(100) ""

}
}
}
}

$elem = simplexml_load_string($xml);
var_dump($elem->count());