MySQLi

<?php

$mysqli = new mysqli($host, $user, $pass, $db);

    $username = $_POST['username'];
    $password = md5($_POST['password']);
    $password2 = md5($_POST['password2']);
    $email = $_POST['email'];

    $stmt = $mysqli->stmt_init();
    $stmt->prepare("INSERT INTO users VALUES (?, ?, ?)");
    $stmt->bindParam('sss', $username, $password, $email);

?>