PHP Captcha file and Javascript together

The form:
<?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
	}
?>
 
 
The Captcha Script:
 
<?php
session_start();
 
/*
* 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);
 
?>