ciao ho questo codice che genera un'immagine e dovrebbe aprirmi una sessione

Codice PHP:
<?php
session_start
();
class 
CaptchaSecurityImages {
    var 
$font 'georgia.ttf';
    function 
generateCode($characters) {
        
/* list all possible characters, similar looking characters and vowels have been removed */
        
$possible '23456789bcdfghjkmnpqrstvwxyz';
        
$code '';
        
$i 0;
        while (
$i $characters) {
            
$code .= substr($possiblemt_rand(0strlen($possible)-1), 1);
            
$i++;
        }
        return 
$code;
    }
function 
CaptchaSecurityImages($width='120',$height='40',$characters='4') {
        
$code $this->generateCode($characters);
        
/* font size will be 75% of the image height */
        
$font_size $height 0.60;
        
$image = @imagecreate($width$height) or die('Cannot initialize new GD image stream');
        
/* set the colours */
        
$background_color imagecolorallocate($image333333);
        
$text_color imagecolorallocate($image15212580);
        
$noise_color imagecolorallocate($image505050);
        
/* generate random dots in background */
        
for( $i=0$i<($width*$height)/3$i++ ) {
            
imagefilledellipse($imagemt_rand(0,$width), mt_rand(0,$height), 11$noise_color);
        }
        
/* generate random lines in background */
        
for( $i=0$i<($width*$height)/150$i++ ) {
            
imageline($imagemt_rand(0,$width), mt_rand(0,$height), mt_rand(0,$width), mt_rand(0,$height), $noise_color);
        }
        
/* create textbox and add text */
        
$textbox imagettfbbox($font_size0$this->font$code) or die('Error in imagettfbbox function');
        
$x = ($width $textbox[4])/2;
        
$y = ($height $textbox[5])/2;
        
imagettftext($image$font_size0$x$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'] > $_GET['characters'] : '4';
$captcha = new CaptchaSecurityImages($width,$height,$characters);
?>
e questo codice nella pagina di invio di una mail che dovrebbe controllare il captcha

Codice PHP:
<?php
                            
echo "codice:".$_SESSION['security_code'];
                        include_once(
'PHPMailer/class.phpmailer.php');
                            
$nome=$_REQUEST['nome'];
                            
$cognome=$_REQUEST['cognome'];
                            
$indirizzo=$_REQUEST['indirizzo'];
                            
$localita=$_REQUEST['localita'];
                            
$cap=$_REQUEST['cap'];
                            
$prov=$_REQUEST['prov'];
                            
$email=$_REQUEST['email'];
                            
$telefono=$_REQUEST['telefono'];
                            
$oggetto_mail=$_REQUEST['oggetto'];
                            
$messaggio=$_REQUEST['messaggio'];
                            
$accettaprivacy=$_REQUEST['accettaprivacy'];
                            
$validazione=$_REQUEST['validazione'];
                            
$validazione_ok=$_SESSION['security_code'];
                            
$indirizzo $indirizzo.' - '.$cap.' '.$localita.'('.$prov.')';
                            if(
$validazione==$validazione_ok){
                                    if(
eregi("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$"$email))
                                    {
                                        if (
trim($email) == ""){
                                            echo 
"<h2>l'e-mail è un campo obbligatorio! - [url='javascript:history.back();']torna[/url]</h2>";
                                        }
                                        elseif (
trim($messaggio) == ""){
                                            echo 
"<h2>il messaggio è un campo obbligatorio! - [url='javascript:history.back();']torna[/url]</h2>";
                                        }
                                        elseif (
trim($accettaprivacy) != "accetto"){
                                            echo 
"<h2>per inviare la mail devi accettare la normativa sulla privacy - [url='javascript:history.back();']torna[/url]</h2>";
                                        }
                                        else {
                                            
$mittente=$email;
                                            if (
$oggetto_mail==""){
                                                
$oggetto="richiesta informazioni dal sito castramaiora.it";
                                            }
                                            else {
                                            
$oggetto=$oggetto_mail;
                                            }
                                            
#
                                            
$corpo=
                                            
#
                                            #
                                            #
                                            # INVIO MAIL                                                                                
                                            #mail("paololosco@gmail.com", $oggetto, $messaggio, $header);
                                            
$mail = new PHPMailer();
                                            
                                            
$mail->From $mittente;
                                            
$mail->FromName "".$nome." ".$cognome."";
                                            
$mail->Subject $oggetto;
                                            
$html_body ='
                                            <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
                                                <head>
                                                <title></title>
                                                </head>
                                                
                                                <body style="margin: 0; padding: 0; text-align: center; width: 100%; background:#000; color:#ccc;">
                                                    <div style="width:550px; margin:0 auto; font-family: Verdana, Arial, Helvetica, sans-serif;">
                                                        <table cellpadding="5" cellspacing="5" width=550 style="border:0; margin: 5px auto;  text-align: left; background: #212121">
                                                            <tr>
                                                                <td colspan="2">
                                                                    <h1 style="font-family: \'Times New Roman\', Times, serif; letter-spacing: -1px; line-height: 24px; font-size:22px; text-align: left; margin: 10px 0 2px 0; color: #987d50; font-weight: normal;">'
.$oggetto.'</h1>
                                                                </td>
                                                            </tr>
                                                            <tr>
                                                                <td width="120" style="padding-top: 30px; text-align: right;">
                                                                    nome:
                                                                </td>
                                                                <td style="padding-top: 30px;">
                                                                    '
.$nome.' '.$cognome.'
                                                                </td>
                                                            </tr>
                                                            <tr>
                                                                <td width="120" style="padding-top: 10px; text-align: right;">
                                                                    email:
                                                                </td>
                                                                <td style="padding-top: 10px;">
                                                                    '
.$email.'
                                                                </td>
                                                            </tr>
                                                            <tr>
                                                                <td width="120" style="padding-top: 10px; text-align: right;">
                                                                    indirizzo:
                                                                </td>
                                                                <td style="padding-top: 10px;">
                                                                    '
.$indirizzo.'
                                                                </td>
                                                            </tr>
                                                            <tr>
                                                                <td width="120" style="padding-top: 10px; text-align: right;">
                                                                    telefono:
                                                                </td>
                                                                <td style="padding-top: 10px;">
                                                                    '
.$telefono.'
                                                                </td>
                                                            </tr>
                                                            <tr>
                                                                <td width="120" style="padding-top: 30px; text-align: right;">
                                                                    oggetto:
                                                                </td>
                                                                <td style="padding-top: 30px;">
                                                                    <h1 style="font-family: \'Times New Roman\', Times, serif; letter-spacing: -1px; line-height: 24px; font-size:22px; text-align: left; margin: 10px 0 2px 0; color: #987d50; font-weight: normal;">'
.$oggetto.'</h1>
                                                                </td>
                                                            </tr>
                                                            <tr>
                                                                <td width="120" style="padding-top: 10px; text-align: right;">
                                                                    richiesta:
                                                                </td>
                                                                <td style="padding-top: 10px;">
                                                                    '
.$messaggio.'
                                                                </td>
                                                            </tr>
                                                        </table>
                                                    </div>
                                                </body>
                                                </html>
                                            '
;
                                            
$mail->MsgHTML($html_body);
                                            
$mail->AddAddress("paololosco@gmail.com");
                                            
$nome_intero=''.$nome.' '.$cognome.'';
                                                if (
$mail->Send())
                                                {
                                                echo 
"<h2>la mail è stata inviata correttamente a</h2>";
                                                
                                                }
                                                else
                                                {
                                                echo 
'Errore: mail non inviata - [url="javascript:history.back();"]torna[/url]';
                                                }
                                            }
                                    }
                                    else{
                                    echo 
"<h2>la mail inserita non è un indirizzo valido! - [url='javascript:history.back();']torna[/url]</h2>";
                                    }
                                }
                                else{
                                echo 
"<h2>il codice di validazione inserito non è corretto! - [url='javascript:history.back();']torna[/url]</h2>";
                                }
    
                        
?>
quello che mi risulta è: se scrivo il codice corretto NON va, se lascio vuoto SI...

ho provato a stampare la sessione e mi da il vuoto...

il bello è che lo stesso identico script, sullo stesso server ma in un altro sito va a meraviglia...

idee?