Description:
------------
`simplexml_load_string()` documentation states

> Returns an object of class SimpleXMLElement with properties containing the data held within the xml document, or FALSE on failure.

So I expect if `FALSE` is returned - it was failed to parse xml, so the `libxml_get_last_error` returns the actual error.

What happens instead - for the empty string the `simplexml_load_string` returns `false` and `libxml_get_last_error` returns previous non actual error.

It should be either a documentation additionally clarify the case, or the implementation behave accordingly.

Test script:
---------------
$previous = libxml_use_internal_errors(true);
$doc = simplexml_load_string('foo');
libxml_use_internal_errors($previous);

$result = libxml_get_last_error();

$previous = libxml_use_internal_errors(true);
$doc = simplexml_load_string('');
libxml_use_internal_errors($previous);

$result = libxml_get_last_error();

var_dump($doc, $result);

Expected result:
----------------
bool(false)
bool(false)

Actual result:
--------------
bool(false)
object(LibXMLError)#2 (6) {
  ["level"]=>
  int(3)
  ["code"]=>
  int(4)
  ["column"]=>
  int(1)
  ["message"]=>
  string(34) "Start tag expected, '<' not found
"
  ["file"]=>
  string(0) ""
  ["line"]=>
  int(1)
}