<?php
error_reporting(E_ALL);
$con = new mysqli('localhost', 'root', '', 'mydb');
if (mysqli_connect_errno()) {
    echo "no connection";
    exit();
}

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    $username = $_POST['username'];
    $email = $_POST['email'];
    $password = $_POST['password1'];

    if(!$sql = $con->prepare("INSERT INTO mytb(username,email,password) VALUES (?,?,?)")){
        print_r($con->error); die;
    }

    $sql->bind_param('sss', $username, $email, $password);

    if($sql->execute()){
        echo 'ok';
    }
    else{
        echo 'not ok';
    }
    header("refresh:2; url=main.php");
}