CAPTCHA-verificatie werkt niet.

<?php
session_start();

class CaptchaSecurityImages {

	var $font = 'monofont.ttf';

	function generateCode($characters) {
		
		$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 = $height * 0.75;
		$image = @imagecreate($width, $height) or die('Kan nieuwe afbeelding niet plaatsen.');
		
		$background_color = imagecolorallocate($image, 255, 255, 255);
		$text_color = imagecolorallocate($image, 20, 40, 100);
		$noise_color = imagecolorallocate($image, 100, 120, 180);
		
		for( $i=0; $i<($width*$height)/3; $i++ ) {
			imagefilledellipse($image, mt_rand(0,$width), mt_rand(0,$height), 1, 1, $noise_color);
		}
		
		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);
		}
	
		$textbox = imagettfbbox($font_size, 0, $this->font, $code) or die('Fout in imagettfbbox functie');
		$x = ($width - $textbox[4])/2;
		$y = ($height - $textbox[5])/2;
		imagettftext($image, $font_size, 0, $x, $y, $text_color, $this->font , $code) or die('Fout in imagettftext functie');
		
		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();
 
class CaptchaSecurityImages {
 
	var $font = 'monofont.ttf';
 
	function generateCode($characters) {
 
		$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 = $height * 0.75;
		$image = @imagecreate($width, $height) or die('Kan nieuwe afbeelding niet plaatsen.');
 
		$background_color = imagecolorallocate($image, 255, 255, 255);
		$text_color = imagecolorallocate($image, 20, 40, 100);
		$noise_color = imagecolorallocate($image, 100, 120, 180);
 
		for( $i=0; $i<($width*$height)/3; $i++ ) {
			imagefilledellipse($image, mt_rand(0,$width), mt_rand(0,$height), 1, 1, $noise_color);
		}
 
		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);
		}
 
		$textbox = imagettfbbox($font_size, 0, $this->font, $code) or die('Fout in imagettfbbox functie');
		$x = ($width - $textbox[4])/2;
		$y = ($height - $textbox[5])/2;
		imagettftext($image, $font_size, 0, $x, $y, $text_color, $this->font , $code) or die('Fout in imagettftext functie');
 
		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);
 
?>

<td colspan="2">
  <img src="security_code.php?width=100&height=40&characters=5" /><br />
		<label for="security_code">Security Code: </label><input id="security_code" name="security_code" type="text" /></td>

<td colspan="2">
  <img src="security_code.php?width=100&height=40&characters=5" /><br />
		<label for="security_code">Security Code: </label><input id="security_code" name="security_code" type="text" /></td>