PHP og Form

Hvad er der lige galt her? Den bliver bare ved med at vise min Form (ingen fejlmeddelelser...)

<?php
if(isset($submit))
{
  $conn = mysql_connect("localhost", "root", "");
  mysql_select_db("testdb", $conn);
  $result = mysql_query("select title1, url1, title2, url2, title3,url3 from url_list where uid = '$username'", $conn);

$num_rows = mysql_num_rows($result);

// if no rows are returned then the username is invalid
    if(!$num_rows)
    {
?>

<html>

<head>
<basefont face="Arial">
</head>

<body>
<center>
<font size="3">User name not found!</font><br>
<a href="dbtest_3.php">Click here to try again.</a>
</center>
</body>
</html>

<?php
    }
   
else
    {
// display list
    $result = mysql_query("select title1, url1, title2, url2, title3, url3 from
url_list where uid = '$username'", $conn);
    list($title1,$url1,$title2,$url2,$title3,$url3) =
mysql_fetch_row($result);

?>

<html>
<head>
<basefont face="Arial">
</head>
<body>
<center>
<font size="3">Welcome, <?php echo $username; ?>!</font><br>
Pick your destination:
<p>
<?php echo "<a href=$url1>$title1</a>"; ?> 
<?php echo "<a href=$url2>$title2</a>"; ?> 
<?php echo "<a href=$url3>$title3</a>"; ?>
</center>
</body>
</html>

<?php
mysql_free_result($result);
}
}
else
{

// $submit not found
// so display a form
?>

<html>

<head>
<basefont face="Arial">
</head>

<body>
<form action="dbtest_3.php" method="POST">
<table>
<tr>
<td>
Username:
</td>
<td>
<input type="text" name="username" length=10 maxlength="30">
</td>
</tr>
<tr>
<td colspan="2" align="center">
<input type="submit" name="submit" value="Log in">
</td>
</tr>
</table>
</form>
</body>
</html>

<?php
}
?>