How to add Capcha to your form

Hi!
Today, I show you how to add capcha to your form.
1. Create file CaptchaSecurityImages.php and Write following code in this file:

<?php
session_start();

class CaptchaSecurityImages {


var $font = 'arial.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 * 0.55;
$image = @imagecreate($width, $height) or die('Cannot initialize new GD image stream');
/* set the colours */
$background_color = imagecolorallocate($image, 25, 25, 25);
$text_color = imagecolorallocate($image, 255, 255, 255);
$noise_color = imagecolorallocate($image, 10, 120, 18);
/* 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);


?>


2. in your form:


<?php
if (isset($_REQUEST['submitAddQuestion']))
{
// capcha
session_start();
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 'Success';
unset($_SESSION['security_code']);
} else 
{
// Insert your code for showing an error message here
echo '
<script type="text/javascript">
alert("Sorry, you have provided an invalid security code!");
</script>
';

                       echo ' 
                               <form action="" method="post">
                               Security Code<br/>
<input class="input-text" style="height:19px;width:155px;" name="security_code" type="text" size="10"/>&nbsp;&nbsp;
<img class="images" border="0px" src="CaptchaSecurityImages.php?width=65&height=20&characters=4" /><br /><br/>
                             <input type="submit" name="submitAddQuestion" value="sent" />
                            </form>

}
}
?>


Thank you for your time. Please feel free to ask any question and feedback.