This code ( a real ugly piece of work) will do it BUT php won't run as a cronjob on my host.
I use a windows scheduler program to hit the script with my browser every day at Midnight.
PHP Code:
<?php
require "global.php";
$today=date("-m-d");
$result = $DB_site->query("SELECT username, email FROM user WHERE birthday like '%$today'");
for ($m=0; $m < mysql_num_rows($result); $m++) {
list($username, $email) = mysql_fetch_row($result);
echo $username . "<br>";
$message = "Happy Birthday $username! Come celebrate your day at [url]http://www.yourforums.com[/url]";
mail ($email,"Happy Birthday!",$message,"From: \"webmaster@yourforums.com\" <You>");
}
?>