php - How to select value from JSON array -

i trying select variable array (at least think it's stored array):

$data = json_encode($response); file_put_contents('file.txt', $data);

gives

"status":200,"response":{  "api_id":"0f42d6be-8ed2-11e3-822e-22135", "bill_duration":36, "call_duration":36, "total_rate":"0.00300"}  }

how can select call_duration value (in php)? i've tried $response['call_duration'], thought should work returns nothing?

$response['call_duration'] correct, think need:

$response['response']['call_duration']

looking @ output after converting json, think original array, $response, looks (in php array format)

$response = array(   'status'=>200,   'response'=>array(     'api_id'=>'0f....etc',     'bill_duration'=>36,      ... etc   ) );

so, need go level deep array call_duration.

KBPS