php生成验证码图片程序

if( isset($_POST['submit'])) {
   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.
  echo 'Thank you. Your message said "'.$_POST['message'].'"';
  unset($_SESSION['security_code']);
   } else {
  // Insert your code for showing an error message here
  echo 'Sorry, you have provided an invalid security code';
   }
} else {
?>

 <form action="form.php" method="post">
  <label for="name">Name: </label><input type="text" name="name" id="name" /><br />
  <label for="email">Email: </label><input type="text" name="email" id="email" /><br />
  <label for="message">Message: </label><textarea rows="5" cols="30" name="message" id="message"></textarea><br />
  <img src="CaptchaSecurityImages.php?width=100&height=40&characters=5" /><br />
  <label for="security_code">Security Code: </label><input id="security_code" name="security_code" type="text" /><br />
  <input type="submit" name="submit" value="Submit" />
 </form>

<?php
 }
?>

/*
* File: CaptchaSecurityImages.php
* Author: Simon Jarvis
* Copyright: 2006 Simon Jarvis
* Date: 03/08/06
* Updated: 07/02/07
* Requirements: PHP 4/5 with GD and FreeType libraries
* Link: http://www.white-hat-web-design.co.uk/articles/php-captcha.php
*
* 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
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details:
* http://www.gnu.org/licenses/gpl.html
*
*/

class CaptchaSecurityImages {

 var $font = 'monofont.ttf';

 function generateCode($characters) {
  /* 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);
   $i++;
  }
  return $code;
 }

 function CaptchaSecurityImages($width='120',$height='40',$characters='6') {
  $code = $this->generateCode($characters);
  /* font size will be 75% of the image height */
  $font_size = $height * 0.75;
  $image = @imagecreate($width, $height) or die('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) or die('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 , $code) or die('Error in imagettftext function');
  /* output captcha image to browser */
  header('Content-Type: image/jpeg');
  imagejpeg($image);
  imagedestroy($image);
  $_SESSION['security_code'] = $code;
 }

}

$width = isset($_GET['width']) ? $_GET['width'] : '120';
$height = isset($_GET['height']) ? $_GET['height'] : '40';
$characters = isset($_GET['characters']) && $_GET['characters'] > 1 ? $_GET['characters'] : '6';

$captcha = new CaptchaSecurityImages($width,$height,$characters);

?>