Recently I started to get a lot of junk in my in-box through the contact form. This was all generated by spam bots on the net so I added in the new feature of human evaluation by use of the image with a number on the contact and comment forms to slow the junk entering my in-box. This technique is known as CAPTCHA (Completely Automated Public Turing to tell Computers from Humans Apart).
A
CAPTCHA test is a program that can generate and grade tests that most humans can pass and current computer programs can't pass.
So this particular one is written with
PHP and
GD (image manipulation part of PHP).
The first part of this is to get a suitably random number and store it somewhere. I have used a session variable so that I can reuse the random function.
function get_random()
{
srand(time());
$max = getrandmax();
$num = rand(1,$max) + rand(1,$max);
$_SESSION['rand'] = $num;
}
Generating the image using that number looks like this, I have saved this in a PHP file and we will look at how to call this in a minute. The code comments should be explanation enough on how it works.
session_start();
//call the function for the random number
$number = get_random();
//assign number to a var
$number = $_SESSION['rand'];
//create a blank image of size 100 x 18
$img_number = imagecreate(100,18);
//set colour vars
$white = imagecolorallocate($img_number,255,255,255);
$black = imagecolorallocate($img_number,0,0,0);
$grey_shade = imagecolorallocate($img_number,255,153,0);
//fill the image with grey shade
imagefill($img_number,0,0,$grey_shade);
//put black rectangle around
ImageRectangle($img_number,0,0,99,17,$black);
//enter the randomnumber string onto image
Imagestring($img_number,9,30,1,$number,$black);
//time to output - set type header to jpeg
header("Content-type: image/jpeg");
//output image
imagejpeg($img_number);
Now because we put the random number in the user Session, it makes it really easy to get a hold of it on processing the form.
$validation = $_SESSION['rand'];
$code = $_POST['code'];
if ($validation != $code){
error("Your validation code was incorrect - please check and try again");
}
