Stored Procedures and Prepared Statements ins PHP

Could you please help me with this: I'm coding PHP and have created a stored procedure in MySQL that receives two input values and return one output parameter

DELIMITER $$

USE `ejemplo`$$

DROP PROCEDURE IF EXISTS `insert_sp`$$

CREATE DEFINER=`root`@`localhost` PROCEDURE `insert_sp`(
    IN ID VARCHAR(8),
    IN Nombre VARCHAR(50),
    OUT msg VARCHAR(50)
    )
BEGIN
    INSERT INTO empleado_php VALUES (ID, Nombre);
    SELECT "Todo bien" INTO msg;
    END$$

DELIMITER ;

and call this via mysqli, everythings works just fine if I use the code snipped below

if (!$mysqli->query("SET @msg = ''") ||
    !$mysqli->query("CALL insert_sp('" . $id . "', '" . $nombre . "', @msg)"))
     echo "CALL failed: (" . $mysqli->errno . ") " . $mysqli->error;

 if (!($res = $mysqli->query("SELECT @msg as _p_out")))
      echo "Fetch failed: (" . $mysqli->errno . ") " . $mysqli->error;

and show the output value with this:

$row = $res->fetch_assoc();
echo $row['_p_out'];

My question is: How am I suppose to code if I have to use prepared statements? could you please give me an example...this is what I' got so far but doesn't work

if (!$mysqli->prepare("SET @msg = ''") ||
         !$mysqli->prepare("CALL insert_sp(?,?,@msg)"))
     echo "CALL failed: (" . $mysqli->errno . ") " . $mysqli->error;

  if (!($res = $mysqli->prepare("SELECT @msg as _p_out")))
     echo "Fetch failed: (" . $mysqli->errno . ") " . $mysqli->error;

  if(!$mysqli->bind_param("ss", $id, $nombre)){
         echo "Error en bind-param ingresar_sp.php " .$mysqli->connect_error;
     }

    $mysqli->execute();

    $row = $res->fetch_assoc();
    echo $row['_p_out'];

By the way...I need to show the output value via echo.

Thanks in advance.

It was very easy, PHP oficial website remarks that you've got to use session variables (MySQL sessions, not PHP), so, the answers to my question is show below:

// bind the value of the first IN parameter to the session variable @id     
$stmt = $mysqli->prepare("SET @id = ?");
$stmt->bind_param('s', $id);
$stmt->execute();

// bind the value of the second IN parameter to the session variable @nombre
$stmt = $mysqli->prepare("SET @nombre = ?");
$stmt->bind_param('s', $nombre);
$stmt->execute();

//bind the value of the OUT parameter
$stmt = $mysqli->prepare("SET @msg = ''");
$stmt->execute();

// execute the stored Procedure
$result = $mysqli->query('call insert_sp(@id, @nombre, @msg)');

// getting the value of the OUT parameter
$r = $mysqli->query('SELECT @msg as _p_out');
$row = $r->fetch_assoc();                       

echo $row['_p_out'];