2 fejl i oprettelse af login script

Jeg er totalt noob når det handler om PHP, og har derfor et par spørgsmål som i nok vil grine af, men ok, lærer det nok. Here it goes:
Jeg har lavet en login (forsøgt) i PHP, og den virker egentlig også, bortset fra 2 fejl.

1)Warning: Failed opening './common_db.inc' for inclusion (include_path='') in c:\appserv\www\login\login.php on line 3

Warning: Cannot send session cache limiter - headers already sent (output started at c:\appserv\www\login\login.php:3) in c:\appserv\www\login\login.php on line 58

Og det betyder?

2)Warning: open(/tmp\sess_093b8f9a64ed7d983a047b0e9ae17a08, O_RDWR) failed: m (2) in c:\appserv\www\login\login.php on line 58

Og det?

Min kode ser således ud:

<?php
//auth_user.php
include "./common_db.inc";
$register_script = "./register.php";

function auth_user($userid, $userpassword) {
    global $default_dbname, $user_tablename;
   
    $link_id = db_connect($default_dbname);
    $query = "SELECT username FROM $user_tablename
                                    WHERE userid = '$userid'
                                    AND userpassword = password('$userpassword')";
$result = mysql_query($query);
if(!mysql_num_rows($result)) return 0;
else {
    $query_data = mysql_fetch_row ($result);
    return $query_data[0];
    }
}

function login_form() {
    global $PHP_SELF;
?>
<!doctype html public "-//W3C//DTD HTML 4.0 //EN">
<html>
<head>
      <title>Helse Login</title>
</head>
<body>
<form method="POST" action="<? echo $PHP_SELF ?>">
    <DIV ALIGN="center"><CENTER>
    <h3>Please log in to access the page you requested.</h3>
<table border="0" width="200" cellpadding="2">
    <tr>
        <th width="18%" align="right" nowrap>ID</th>
        <td width="82%" nowrap>
            <input type="text" name="userid" size="8">
        </td>
    </tr>
    <tr>
        <th width="18%" align="right" nowrap>Password</th>
        <td width="82%" nowrap>
            <input type="text" name="userpassword" size="8">
        </td>
    </tr>
    <tr>
        <td width="100%" colspan="2" align="center" nowrap>
            <input type="submit" value="LOGIN" name="submit">
        </td>
    </tr>
</table>
</center></DIV>
</form>
</body>
</html>
<?
}
session_start();
if(!isset($userid)) {
        login_form();
        exit;
    }
    else {
        session_register("userid", "userpassword");
        //$username = auth_user{$userid, $userpassword);
        if(!$username) {
            session_unregister("userid");
            session_unregister("userpassword");
            echo "Authorization failed. " .
                "You must enter a valid userid and password. " .
                "click the following link to try again.<br>\n";
        echo "<A HREF=\"$register_script\">Membership</A>";
          exit;
        }
        else echo "Welcome, $username!";
    }
?>

Håber der nogen der kan hjælpe mig med mine problemer.

Mazey