Captcha...

<?php phpinfo(); ?>

extension=php_gd2.dll

extension=php_gd2.so

<?php
session_start();

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);

?>

<?php
session_start();

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
   }
?>

<?php
var_dump(gd_info());
?>