PHP Example
Screen shot
PHP Captch Code Script

<?php
$x=array("a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v",
"w","x","y","z","A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T",
"U","V","W","X","Y","Z","0","1","2","3","4","5","6","7","8","9");

$str='';

for($i=0;$i<5;$i++)
{
$str=$str.$x[rand(0,count($x)-1)];
}

$im=imagecreate(80, 40);
$background_color = imagecolorallocate($im, 139, 69, 19);
$text_color = imagecolorallocate($im, 255, 255, 255);
imageline($im, 0, 10, 80,60 ,$text_color);
imageline($im,70, 0, 0,70 ,$text_color);
imagestring($im, 5, 12, 10,  $str, $text_color);
header("Content-type: image/png");
imagepng($im);
imagedestroy($im);
?>

You can download source code here