Well.. I am getting an Undefined index notice from the Log-In code below.
I have not written the code myself, so I guess the problem is that I am not sure what "op" is exactly.
Can someone please provide me with some input on what I need to do in order to get rid of the 'Undefined index' notice?

Notice: Undefined index: op in C:\program files\apache\htdocs\login.php on line 6

PHP Code:

<?php
session_start();
$conn = mysql_connect("localhost","hat","pas");
$db = mysql_select_db("database");

    if ($_GET["op"] == "login")
        {
        
    $username = $_POST["username"];
    $password = $_POST["password"];
        if (!$username || !$password)
            {
            die("You need to supply a username and password.");
            }
            
    $result = MYSQL_QUERY("SELECT * from users WHERE username='$username'and password=PASSWORD('$password')")
           or die ("Name and password not found or not matched");

    $worked = mysql_fetch_array($result);

            $id = $worked['id'];
            $_SESSION["valid_id"] = $id;
            $_SESSION["valid_user"] = $username;
            $_SESSION["valid_time"] = time();

            // Redirect to member page
            Header("Location: members.php");
        }
    else
        {
        echo "<form action=\"?op=login\" method=\"POST\">";
        echo "Username: <input name=\"username\" size=\"15\"><br />";
        echo "Password: <input type=\"password\" name=\"password\" size=\"8\"><br />";
        echo "<input type=\"submit\" value=\"Login\">";
        echo "</form>";
        }
?>

You should do it like this:

PHP Code:

if (isset($_GET['op'])) {
   if ($_GET['op'] == 'login') {
      // ... blah-blah...
   }
} 


Or just add error_reporting (E_ALL ^ E_NOTICE) in the beginning of your script. Also you can read http://php.net/error_reporting for more clues on it.

Thanks for the help!