<?php

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

    try {
        $pdo = new PDO('mysql:host=host;dbname=name', 'username', 'password');
    } catch (PDOException $e) {
        exit($e->getMessage());
    }

    $username = $_POST['username'];
    $email = $_POST['email'];
    $password = $_POST['password'];
    $password2 = $_POST['password2'];

    if (empty($username)) {
        $error = 'username is empty';
    } elseif (empty($email) || filter_var($email, FILTER_VALIDATE_EMAIL)) {
        $error = 'email must be a valid email address';
    } elseif (empty($password)) {
        $error = 'you must enter a password';
    } elseif (strcmp($password, $password2) !== 0) {
        $error = 'your repeat password must be the same as your password';
    } else {
        $selectSql = $pdo->prepare('SELECT COUNT(*) FROM users WHERE username = ? OR email = ?');
        $selectSql->execute(array($username, $email));
        if($selectSql->fetchcolumn(0) == 0) {
            $insertSql = $pdo->prepare('INSERT INTO users VALUES (null, ?, ?, ?)');
            if ($insertSql->execute(array($username, $email, password_hash($password, PASSWORD_BCRYPT)))) {
                $success = 'user has been created';
            } else {
                $error = 'error creating the user';
            }
        } else {
            $error = 'username or email is taken';
        }
    }

}

?>

<form method="post">
    <div>
        <?=isset($error) ? "Error! {$error}" : ''?>
        <?=isset($success)  ? "Success! {$success}" : ''?>
    </div>
    <input type="text" name="username" placeholder="username">
    <input type="email" name="email" placeholder="email">
    <input type="password" name="password" placeholder="password">
    <input type="password" name="password2" placeholder="retype password">
    <input type="submit" name="btnSubmit" value="Register">
</form>