<?php

if(isset($_POST['formsub'])) {
    $username = $_POST['user'];
    $password = $_POST['pass'];

    $pdo = new PDO("mysql:host=localhost;dbname=database", 'username', 'password');
    $query = $pdo->prepare("INSERT INTO users (user,pass) VALUES (:user,:pass)");
    $query->bindValue(":user", $username, PDO::PARAM_STR);
    $query->bindValue(":pass", $password, PDO::PARAM_STR);
    $query->execute();
}

?>

<form action="index.php" method="post">
username: <input type="text" name="user"><br>
password: <input type="password" name="pass"><br>
<input type="hidden" name="formsub" value="1">
<input type="submit">
</form>