The connection variable in mysqlli_query is not defined

I have the following problem is that $con is not defined, despite i use it to connect to the database. I don't understand why is that.

<?php

    define("DB_HOST", "localhost");
    define("DB_NAME", "Bar_buddy_users");
    define("DB_USER", "local_server");
    define("DB_PASSWORD","010203");
    error_reporting( E_ALL );
    ini_set('display_errors', 1);
    $con = new mysqli(DB_HOST,DB_USER,"010203",DB_NAME) or die("Failed to connect to database");
    if ($con->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    }
    echo "Connected successfully";
    function NewUser(){

        $first_name= $_POST["firstname"];
        $last_name = $_POST["lastname"];
        $email = $_POST["email"];
        $password = $_POST["password"];

        $query = "INSERT INTO Users (firstname, lastname, email, password) VALUES ('$first_name', '$last_name', '$email', '$password')";
        mysqli_query($con,$query);
        mysqli_close($con);


    }

    ?>