CAPTCHA is used on many website to ensure that the response is not generated by computer. The term “CAPTCHA” was based upon the word capture. It is a contrived acronym for Completely Automated Public Turing test to tell Computers and Humans Apart. A common type of CAPTCHA requires that the user type letters or digits from a distorted image that appears on the screen. (Wikipedia).

Generating Captcha Image Using PHP

PHP allows us to create and/or manipulate image using it’s GD Library. In this article, I will show you how to generate a simple captcha image and use it as a form validation. You can learn more about manipulating image using PHP’s GD Library from the official site.

<?php

/* PHP Captcha Image Generator by Arie Putranto
    http://arie.putranto.com/blog/php-script/php-captcha-image-verification.html/ */

session_start();

// Font configuration
$font_size = 16;
$font_file = 'cour.ttf';
$string_length = 5;

// Calculating image dimension based on font configuration
$img_height = $font_size * 2;
$img_width = ($string_length * $font_size) + $font_size;

// Define the code for showing on the image
$_SESSION['validation'] = $string = (isset($_SESSION['code'])) ? substr($_SESSION['code'],0,$string_length) : strtoupper(substr(md5(rand(00000,99999)),0,5));

// Generating the captcha image
$image = @imagecreatetruecolor($img_width, $img_height);

// Define image colors
$bg_color = @imagecolorallocate($image, 252, 252, 252);
$border_color = @imagecolorallocate($image, 200, 200, 200);
$string_color = @imagecolorallocate($image, 100, 100, 100);

// Drawing the image backgroung and borders
@imagefill($image,0,0,$bg_color);
@imageline($image,0,0,0,$img_height,$border_color);
@imageline($image,$img_width-1,0,$img_width-1,$img_height,$border_color);
@imageline($image,0,0,$img_width-1,0,$border_color);
@imageline($image,0,$img_height-1,$img_width-1,$img_height-1,$border_color);

// Write the verification code into the image
for($i=0;$i<$string_length;$i++) {
 @imagettftext($image,$font_size,0,($font_size/2)+2+($font_size*$i),$img_height-($img_height/3)+1,$string_color,$font_file,$string[$i]);
}

// Load the image into the browser
header("Expires: Sun, 1 Jan 2000 12:00:00 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/jpeg");

@imagejpeg($image, NULL, 100);
@imagedestroy($image);

?>

The generated CAPTCHA image will look like this:

Using the captcha image in a form validation

To do that, we need to add a few PHP line on the form and save the form as a PHP file. Check out the code below:

<?php 

session_start();

// Generating the verification code for the CAPTCHA image
$_SESSION['code'] = strtoupper(md5(rand(00000,99999)));

// Processing the form request
if(isset($_POST['captcha'])) {
 $posted_captcha = stripslashes($_POST['captcha']);
 $validation = $_SESSION['validation'];
 if($posted_captcha != $validation) print 'Invalid captcha verification';
 else print 'Captcha code is verified';
} else {
 print 'Insert the number below (<em>Case sensitive</em>)';
}

?>
<img src="./captcha.php" alt="" />

<form name="test" method="post" enctype="multipart/form-data" action="">
<input type="text" name="captcha" value="" />
<input type="submit" value="Submit" />
</form>

Save it as a PHP file. To use the above scripts, simply upload them to the same directory along with the font file. You might need to style the form a bit to match your web design.