Here is a simple CAPTCHA image generated via PHP URL ...
<?php
session_start();
// create a random string
$md5_hash = sha1(md5(uniqid(null, true)));
// create a 5 character security code
$security_code = substr($md5_hash, 15, 5);
$_SESSION['captcha_code'] = $security_code;
// set width and height
$width = 72;
$height = 25;
// create image
$image = imagecreate($width, $height);
$white = imagecolorallocate($image, 255, 255, 255);
$black = imagecolorallocate($image, 0, 0, 0);
// setup the image background
imagefill($image, 0, 0, $black);
// add string to the image
imagestring($image, 3, 21, 7, $security_code, $white);
imagerectangle($image,0,0,$width-1,$height-1,$grey);
// setup the page headers
header("Content-Type: image/jpeg");
// output the image to the browser
imagejpeg($image);
<img src="captcha.php" />
So, using the code above, we could alter it to display/generate a different image based on the $_GET values supplied ... i.e.
$toGet = $_GET['FARMVILLE'];
// sanatize data
// get image to display from the DB
// assign image to output variable and/or output the image
