Only echo works in this code in this if statement (PHP)

if(isset($_POST['submit'])) {

            $username = strtolower($_POST['username']);

            $password = hash('sha512', $_POST['password']);

            if($username && $password){
                while($data = mysqli_fetch_array($rs)){
                    if($username == $data[1]){
                        if($password == $data[2]){
                            session_start();
                            $_SESSION['loggedin'] = true;
                            $_SESSION['username'] = $username;

                            $exp = time() + (60*60*24*365);
                            setcookie('user', $username, $exp);
                            echo "test";
                            header("location: dash.php");
                            exit();
                        } else {
                            error("Incorrect Password");
                        }
                    }
                }
            } else {
                error("Please insert your username and password");
            }
        }

In this code, the content of the if statement ( if($password == $data[2]) ) doesn't run correctly: only the echo function works, all the other doesn't!

Is there a solution to this issue?

I use PHP 7.0