how to create easy captch in php and validation server side

stp1-create CaptchaSecurityImages.php

<?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);

?>

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stp2-
    <img src="CaptchaSecurityImages.php?width=162&height=45&characters=5" />

    <input type="text" name="captcha" value="" class="txtinput" />

stp3-for validate in php

 $securetycheck = $_POST['captcha'];
if(empty($securetycheck))
    {
        $err_msg = "Please Enter Security Code";
    }

    else if($_POST['captcha']!=$_SESSION['security_code']){
       // echo "====".$_SESSION['security_code']."==".$_POST['captcha'];
        $err_msg = "Please Enter Proper Security Code";

    }