Using the second argument (arr) doesn't seem to work. No elements will be inserted into the array by parse_str, and variables will still be created in the current scope.

Example:

<?php
  $querystring = "foo=bar&bar=foo";
  parse_str($querystring, $a);

  var_dump($a);
?>

Will output "NULL", and:

<?php
  var_dump($foo);
?>

...will output "bar".