Normally, PHP variable names can’t start with a digit. You can’t access $data as an array either as stdClass does not implement ArrayAccess — it’s just a normal base class.
However, in cases like this you can try accessing the object attribute by its numeric name like so:
echo $data->{'0'}->UserName;
The only reason I can think of why Spudley’s answer would cause an error is because you’re running PHP 4, which doesn’t support using foreach to iterate objects.