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Creating a Captcha

Captchas are used a lot nowadays for things such as user registrations to try and make sure that it is a human filling out the form and not a bot/script.

This is how to make a simple one:

In this script where there is a link to "image.jpeg" I have used this image: http://blog.nielsendata.com/image.axd?picture=2008%2F10%2FcaptchaBackground.jpg So you may want to link directly to that, or find your own background image to use.

<?php
// Set the content-type
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/png'); // Force it to be displayed as an image

class Captcha
{

    private $chars;
    public $min_len;
    public $max_len;
    public $bg_img;
    private $chars_text;
    private $chars_num;
    private $chars_both;

    public function __construct($c)
    {
        $this->chars = $c; // Could be either: "a-z", "0-9", "both"
        $this->min_len = 4; // Minimum length of captcha string
        $this->max_len = 6; // Maximum length of captcha string
        $this->bg_img = "image.jpeg";

        $this->chars_text = "abcdefghijklmnopqrstuvwxyz";
        $this->chars_num = "0123456789";
        $this->chars_both = $this->chars_num . $this->chars_text . $this->chars_num;

    }

    public function draw()
    {
        
        // Create the image
        $im = imagecreatefromjpeg($this->bg_img);

        // Colour to write in (r,g,b)
        $col = imagecolorallocate($im, 0, 0, 0);

        // Create text to write
        switch($this->chars)
        {
            case 'a-z':
                $text = $this->chars_text;
            break;
            case '0-9':
                $text = $this->chars_num;
            break;
            default:
                $text = $this->chars_both;
            break;
        }

        $text = $this->generateRandomString($text);
        
        // Specify Font Size
        $font = 5; // 1-5

        // Specify x and y co-ords on image
        $x = rand(1,50);
        $y = rand(1,20);

        // Add the text
        imagestring($im, $font, $x, $y, $text, $col);

        // Draw Image
        imagepng($im);
        imagedestroy($im);
         


    }

    public function generateRandomString($list)
    {

    $key = '';
    $length = rand($this->min_len, $this->max_len);

     for($i=0;$i<$length;$i++)
     {
     $key .= $list{rand(0,strlen($list)-1)};
     }

    return $key;

    }

}

$myCaptcha = new Captcha("a-z");
$myCaptcha->draw();
?>

You could then link to this file like you would an image, e.g.

<img src="image.php" alt="Captcha" />

See example below (click image to refresh it):



Additional things you may want to think about include:

• Different fonts
• Distractions - Other smaller characters to confuse bots
• Better backgrounds
• Automatically generated backgrounds (e.g. drawing random lines and shapes on the background)

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