simplexml_load_string结果不一致

This is strange and I'm struggling to figure out how to solve it.

$xml = '<?xml version="1.0" encoding="utf-8"?><myinfo><myname>Foo</myname></myinfo>';
$return_data_obj = simplexml_load_string($xml);
echo "ARRAY: <pre>" . print_r((array)$return_data_obj, 1) . "</pre>";

Results are inconsistent ... Hit F5 for refresh, I see the array, Hit F5 again, good ... results. Hit F5 empty array, again F5, empty again, then F5 array show sup and so on. No code in change, just simple refresh. I've tried everything i could find, adding extra parms in simplexml_load_string, namespace, etc - nothing helped.

Anyone have any idea?

NOTE: I'm on PHP 5.3.2

I tried this code and can simply print the array using

echo "ARRAY: <pre>" . print_r((array)$return_data_obj, 1) . "</pre>";