Use json_decode() to create array insead of an object

I am trying to decode a JSON string into an array but i get the following error.

Fatal error: Cannot use object of type stdClass as array

Here is the code:

$json_string = 'http://www.example.com/jsondata.json';

$jsondata = file_get_contents($json_string);
$obj = json_decode($jsondata);
print_r($obj['Result']);