There is no space before <?session_start(); ...
(one version of) the php code is the following...
<?session_start();
$valid_user=$ID;
session_register("valid_user");
?>
<html>
<head>
<title>Processing Login</title>
</head>
<body>
<center>
<img src = "logo.gif" alt = "Logo"/>
</br>
<?php
$db = mysql_pconnect("localhost","bla bla ","bla bla") or die ("Could not Connect");
mysql_select_db("bla bla")or die("Could not select database");
//$ID and $password comes from a previous form
$query="SELECT Fname FROM user WHERE login = '$ID' AND password='$password' AND type='MEMBER' " ;
$result=mysql_query($query)or die("COULD NOT EXECUTE FIRST QUERY");
$num_results = mysql_num_rows($result);
if($num_results == 0)
{
session_destroy();
?>
<h1>Invalid user name or password</h1>
</br><p>Click <a href = "login_form.html">HERE</a>to try again</p>
</br><p>Click <a href = "sendPassword.html">HERE</a>if you forgot your password</p>
</br><p>Click <a href = "non_member.php4">HERE</a>to enter the non-member area of this site</p>
<?
}
else
{
?>
<h1>Welcome <?=$memberName?>,</h1>
</br><p>You may click <a href ="member.php4">HERE</a>to enter the member's section.</p>
</br>
<?
}
?>
</center>
</body>
</html>
