Storing Session values for captcha's

I'm trying to create a captcha code but the code that is being created isn't being stored in the session variable. Here is the file I'm using to create the code and store the value. I have a session_start() on my php page that will display the captcha as well.

开发者_StackOverflow<?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 */
    $_SESSION['security_code'] = $code;
    $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);
}

}

$width = isset($_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);

?>

I can store other items in sessions and they work just fine. I'm not sure why this value won't store. I've found other posts about getting captcha's working but nothing about issues with storing the value in sessions. Any suggestions?

Thanks Robert

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I did end up using reCAPTCHA to accomplish this task.

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How is your image being loaded into the user's browser? Is it on the same domain? If it is not, the problem most likely is that your cookie is being excluded because it's a "3rd party cookie." To get the browser to save this cookie, you have to set a P3P header, so the browser knows it's okay to save the cookie.

An example of a valid P3P header would be:

header('P3P: CP="CAO PSA OUR"');

Hope this helps.