To add current clients - put the actual values you want to store in the values(...) bit. (password function encrypts)
mysql> insert into client (username,password, siteurl) values ("username", password("pwd"), "url");
To check valid password and transfer to site
$sql = "select * from client where client.username = $fm_username and ".
"client.password = password($fm_password))";
$result = mysql_query($sql);
if ($row = mysql_fetch_array($result)) {
$url = $row["siteurl"];
header("location: $url");
exit();
} else {
header("location: "login.html");
exit();
}