php - json_encode(): Invalid UTF-8 sequence in argument -

i cannot of following code work statements array working with.

if (!$allowed)         {             $output = array('success'=>false, 'message'=>'insufficient user access', 'args'=>$_post);         }         else         {             $output = isset($_post['data']) ? $output = call_user_func(array($controller, $_post['request']), $_post['data']) : call_user_func(array($controller, $_post['request']));         }  echo $output = $output ? json_encode($output) : json_encode(false); 

this last statement generating error. have tried mb_chekc_encloding , iconv. cannot either of these statements work making $output show.