$arr = array('a','b');
$arrJsonDecode = json_decode($arr,true);
var_dump($arr);
echo '-----------'.PHP_EOL;
var_dump($arrJsonDecode);
echo '-----------'.PHP_EOL;
array(2) {
  [0] =>
  string(1) "a"
  [1] =>
  string(1) "b"
}
-----------
NULL
-----------