Description:
------------
If I try to use variable with non-string name (e.g. 
$x = 10; $$x = ...) the name is converted to a string using standard conversion rules. That's ok.
But this doesn't work on object's field which is IMHO a bug and it implies some a buggy behavior.
See the code bellow.

It seems that by:
$x = null;
$a->$x = "whatever";
one can somehow create a private variable (or something like that, don't know what ":private" means)!

Moreover, there is possibly a bug in get_object_vars when a field name is a numeric string (e.g. "10") (compare the first item of results of get_object_vars and var_dump).

Reproduce code:
---------------
function field_names_test()
{  
  $a = new stdClass();
  
  $x = 10;
  $a->$x = "int(10)";
  
  $x = "10";
  $a->$x = "string('10')";
  
  $x = "";
  $a->$x = "string('')";
  
  $x = null;
  $a->$x = "null";
  
  $x = 1.8;
  $a->$x = "double(1.8)";
  
  $x = false;
  $a->$x = "bool(false)";
  
  $x = array(1,2,3);
  $a->$x = "array(1,2,3)";
  
  var_dump(get_object_vars($a));
  var_dump($a);
}
field_names_test();

Expected result:
----------------
array(4) {
  ["10"]=>
  string(12) "string('10')"
  [""]=>
  string(11) "bool(false)"
  ["1.8"]=>
  string(11) "double(1.8)"
  ["Array"]=>
  string(12) "array(1,2,3)"
}
object(stdClass)#1 (4) {
  ["10"]=>
  string(12) "string('10')"
  [""]=>
  string(11) "bool(false)"
  ["1.8"]=>
  string(11) "double(1.8)"
  ["Array"]=>
  string(12) "array(1,2,3)"
}


Actual result:
--------------
array(3) {
  [10]=>
  string(12) "string('10')"
  ["1.8"]=>
  string(11) "double(1.8)"
  ["Array"]=>
  string(12) "array(1,2,3)"
}
object(stdClass)#1 (4) {
  ["10"]=>
  string(12) "string('10')"
  [":private"]=>
  string(11) "bool(false)"
  ["1.8"]=>
  string(11) "double(1.8)"
  ["Array"]=>
  string(12) "array(1,2,3)"
}