Log in function not working

function login($email, $password, $mysqli){
    if ($stmt = $mysqli->prepare("SELECT id, username, password, salt
        FROM members
        WHERE email = ?
        LIMIT 1")){
        $stmt->bind_param('s', $email);
        $stmt->execute();
        $stmt->store_result();

        $stmt->bind_result($user_id, $username, $db_password, $salt);
        $stmt->fetch();

        //hash the password with unique salt
        //$password = hash('sha512',$password . $salt);
        if($stmt->num_rows ==1){
            if(checkbrute($user_id, $mysqli) == true){
                return false;
            } else{
                if($db_password == $password){
                    $user_browser = $_SERVER['HTTP_USER_AGENT'];
                    session_start();
                    $user_id = preg_replace("/[^0-9]+/", "", $user_id);
                    $_SESSION['user_id'] = $user_id;
                    $username = preg_replace("/[^a-zA-Z0-9_\-]+/",
                                                                "",
                                                            $username);
                    $_SESSION['username'] = $username;
                    $_SESSION['login_string'] = hash('sha512', $password. $user_browser);
                    return true;
                } else{

                    $now = time();
                    $mysqli->query("INSERT INTO login_attempts(user_id, time)
                    VALUES('$user_id', '$now')");

                    echo mysli_error($mysqli);
                }
            }
        }else{
            //No user exists.
            //return false;
            echo mysqli_error($mysqli);
        }
        }
}