PHP bind_param not defined

Question

I am working in MAMP trying to make a login function.
My connection code is:

$servername = "localhost";
$username = "root";
$password = "root";
$db = "world";

$mysqli = new mysqli($servername, $username, $password, $db);

if($mysqli->connect_error){
    die("Connection failed: " . $conn->connect_error);
}

My login function:

if (isset($_POST['email'], $_POST['p'])){
$email = $_POST['email'];
$password = $_POST['p']; //hashed password

if(login($email, $password, $mysqli) == true){
    header('Location: ../protected_page.php');
    }else{
    echo 'failed login';
    }
}
function login($email, $password, $mysqli){
if($stmt = $mysqli->prepare("SELECT USERID, USERNAME, PASSWORD, SALT FROM USERS WHERE EMAIL = ? LIMIT 1")){
    $stmt = $mysqli->bind_param('s', $email);
    $stmt->execute();
    $stmt->store_result();
    $stmt->bind_result($user_id, $username, $db_password, $salt);
    $stmt->fetch();
}

Error:

[17-Oct-2015 08:46:06 Europe/Berlin] PHP Fatal error: Call to undefined method mysqli::bind_param() in /Users.../Site/include/functions.php on line 24

Answer 1

You need to call bind_param for your statement, like this:

$stmt->bind_param('s', $email);

You have already defined $stmt one line before that. $mysqli does not have a bind_param function.