用php制作一个日志文件

我的注册页面看起来像这个

$checkForUser = mysql_query("SELECT id FROM `users` WHERE `username`='{$posts['username']}' OR `email`='{$posts['email']}'");
$checkForUserRows = mysql_num_rows($checkForUser);
$ip = VisitorIP();
$checkForIP = mysql_query("SELECT id FROM `users` WHERE `ip`='{$ip}'");
$checkForIPRows = mysql_num_rows($checkForIP);
if ($checkForUserRows > 0) {
    $error = "Username or email already registered!";
}else if ($checkForIPRows > 0)  {
    $error = "You may only have one account! This attempt has been logged. You will be banned! ";

我想自动禁止ip已经存在的帐户，并出现以下错误$error = "You may only have one account! This attempt has been logged. You will be banned! ";