Very basic code demonstrating the use of the GD library by creating a simple CAPTCHA.
[code lang="PHP"] function randomString($length){
$pattern = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
if(empty($length)){
$length = "10";
}

for($i=0; $i<$length; $i++){
$string .= $pattern{rand(0,61)};
}

return $string;
}

function captcha() {
header("Content-type: image/png");
$im = @imagecreate(100, 50) or die("Cannot Initialize new GD image stream");
$background_color = imagecolorallocate($im, 0, 0, 0);
$text_color = imagecolorallocate($im, 255, 0, 0);
$string = randomString();
imagestring($im, 10, 5, 5, $string, $text_color);
imagepng($im);
imagedestroy($im);
}

captcha();
?>[/code]

captcha gd PHP