change the string to array is
$string = '[{"name":"jack","address":"who knows"},{"name":"jill","address":"who knows too"}]';
$array = json_decode($array,true);
but what if it's reversed change array to string
$array = [{"name":"jack","address":"who knows"},{"name":"jill","address":"who knows too"}];
$string = $array; // $string is $string like on first example