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PHP Captcha Security Images

Posted on: May 30th, 2009 by Raez Mon No Comments

Have you ever wondered about what are captchas and what are their uses? This script generates images (known as “Captcha’s”) which contain security codes used for protecting a form from spam bots. By encoding a “password” inside an image and asking the user to re-enter what they see you can verify the user is a human and not automated software submitting your form. Have a screenshot of what we  are going to develop.

Difficulty : Easy | Level : Beginner | Time < 15minutes


how to create captcha

The code

< ?php
 /* * File: CaptchaSecurityImages.php 
 * Author: Mohammed Aqeel 
 * Copyright: 2009 Team Webgalli 
 * Date: 03/08/06 
 * Updated: 07/02/09 
 * Requirements: PHP 4/5 with GD and FreeType libraries 
 * Link: 
 * This program is free software; you can redistribute it and/or 
 * modify it under the terms of the GNU General Public License 
 * as published by the Free Software Foundation; either version 2 
 * of the License, or (at your option) any later version. 
 * This program is distributed in the hope that it will be useful, 
 * but WITHOUT ANY WARRANTY; without even the implied warranty of 
 * GNU General Public License for more details: 
	var$font = 'monofont.ttf'; 
	/* list all possible characters, similar looking characters and vowels have been removed */
	$possible = '23456789bcdfghjkmnpqrstvwxyz'; 
	$code = ''; $i = 0;
	while($i < $characters){
		$code .= substr($possible, mt_rand(0, strlen($possible)-1), 1); 
		$code = $this->generateCode($characters); 
		/* font size will be 75% of the image height */
		$font_size = $height * 0.75; 
		$image = imagecreate($width, $height)ordie('Cannot initialize new GD image stream'); 
		/* set the colours */
		$background_color = imagecolorallocate($image, 255, 255, 255); 
		$text_color = imagecolorallocate($image, 20, 40, 100); 
		$noise_color = imagecolorallocate($image, 100, 120, 180); 
		/* generate random dots in background */
		for($i=0; $i< ($width*$height)/3; $i++ ){
			imagefilledellipse($image, mt_rand(0,$width), mt_rand(0,$height), 1, 1, $noise_color);
		/* generate random lines in background */
		for($i=0; $i<($width*$height)/150; $i++ ){
			imageline($image, mt_rand(0,$width), mt_rand(0,$height), mt_rand(0,$width), mt_rand(0,$height), $noise_color); 
		/* create textbox and add text */
		$textbox = imagettfbbox($font_size, 0, $this->font, $code)ordie('Error in imagettfbbox function'); 
		$x = ($width - $textbox[4])/2; 
		$y = ($height - $textbox[5])/2; 
		imagettftext($image, $font_size, 0, $x, $y, $text_color, $this->font , $code)ordie('Error in imagettftext function'); 
		/* output captcha image to browser */
		header('Content-Type: image/jpeg'); 
		$_SESSION['security_code'] = $code; 
	$width = isset($_GET['width']) && $_GET['height'] < 600 ? $_GET['width'] : '120'; 
	$height = isset($_GET['height']) && $_GET['height'] < 200 ? $_GET['height'] : '40';
	$characters = isset($_GET['characters']) && $_GET['characters'] > 2 ? $_GET['characters'] : '6';
	$captcha = newCaptchaSecurityImages($width,$height,$characters);


Copy and paste the above code and save it on your webserver as CaptchaSecurityImages.php. You will also need to place a copy of the “Monofont” font in the same directory as the CaptchaSecurityImages.php file. (Alternatively you can replace the line var $font = ‘monofont.ttf’; with the name of whatever font you want to use)

You can  download the which contains all the files needed to implement the script including the required font.

Place the following code on your form. This will generate an image with a random string of characters along with the text field where the user will retype the code.

 Security Code:

You can also specify certain options for the image by passing them as variables to CaptchaSecurityImages.php. The options available are the width and height of the image and the number of characters


Place the following in the code where the form is submitted to. This code will check what the user has typed matches the code in the image.

< ?php
	if(($_SESSION['security_code'] == $_POST['security_code']) && (!empty($_SESSION['security_code']))){
		// Insert you code for processing the form here, e.g emailing the submission, entering it into a database. 
		// Insert your code for showing an error message here

Optional Extras

If you would rather your tag links to a jpg rather than a php file you can use mod_rewrite. By inserting the following in your .htaccess file you can use instead.

RewriteEngine on
RewriteRule captcha.jpg /CaptchaSecurityImages.php

You may wish to change the colour of the captcha image, this can be done by editing the background_colour, text_colour and noise_colour variables. The imagecolorallocate() function constucts a colour from the given RGB (red, green and blue) values, each of these is a number between 0-255. Another idea you might want to try is using the mt_rand function to randomize the colour each time a captcha is generated.

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