image validate

[brown2005]

<?php

session_start();

$rand = rand(100000, 999999);

$_SESSION['image_random_value_1'] = md5($rand);

$image = imagecreate(60, 30);

$bgColor = imagecolorallocate ($image, 255, 255, 255);

$textColor = imagecolorallocate ($image, 0, 0, 0);

imagestring ($image, 5, 5, 8, $rand, $textColor);

header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");

header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");

header("Cache-Control: no-store, no-cache, must-revalidate");
header("Cache-Control: post-check=0, pre-check=0", false);

header("Pragma: no-cache");

header('Content-type: image/jpeg');

imagejpeg($image);

imagedestroy($image);

?>

hi i have the above code which makes an image of a number which must be copied to process the register.php form, but what i want to do is add letters, both capitals and none capitals into it.

can anyone help please?