php return json data

//code igniter
$query="qry";
$query = $this->db->query($query);
$res=$query->result();
return json_encode($res);

header('Content-type: application/json');
echo json_encode($array);

  <?php
$myObj->name = "John";
$myObj->age = 30;
$myObj->city = "New 
  York";

$myJSON = json_encode($myObj);

echo $myJSON;
?>
 

header('Content-Type: application/json'); 

$colors = array("red","blue","green");
echo json_encode($colors);