<?php
$sql = new mysqli('localhost', 'root', '', 'sitenok');
if($sql->connect_errno)
	return printf("MySQL Error #%d: %s", $sql->connect_errno, $sql->connect_error);
if(isset($_POST['submit']))
{
	$login = $sql->real_escape_string($_POST['login']);
	$password = $sql->real_escape_string($_POST['password']);
	if($password == $_POST['confirm'])
	{
		if(!$q = $sql->query(sprintf("SELECT * FROM `register` WHERE `login` = '%s'", $login)))
			return printf("MySQL Query Error #%d: %s", $sql->errno, $sql->error);
		if($q->num_rows)
		{
			if(!$q = $sql->query(sprintf("INSERT INTO `register` (`login`, `password`) VALUES ('%s', '%s')", $login)))
				return printf("MySQL Query Error #%d: %s", $sql->errno, $sql->error);
			printf("Вы успешно зарегистрировались!");
		} else
			printf("Такой пользователь уже существует<br />\n");
	} else
		printf("Пароли не совпадают<br />\n");
}
?>
<form method='post'>
	Username: <input type='text' name='login' value='<?=$_POST['login']?>' required /><br />
	Password: <input type='password' name='password' required /><br />
	Confirm the password: <input type='password' name='confirm' required /><br />
	<button type='submit' name='submit'>Sign up</button>
</form>