Problem with sessions in combination with external php file

Hello,

I'm already searching for a solution a couple of hours.

Maybe you can/will help me.

I'm created my site in modx but I have an external php(captcha) file where I'm using session variables.

this is my external captcha php file:
<?php
//require_once ($_SERVER["DOCUMENT_ROOT"] . '/manager/includes/config.inc.php');
//startCMSSession();
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 */
$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);

?>








And this is my snippet that I'm using: the 'CaptchaSecurityImages.php' returns an image with the code


session_start();
$output ="<form id='loginForm' name='loginForm' method='POST' action='[~[*id*]~]'>
<table border='0' align='center' cellpadding='2' cellspacing='1'>

<tr>
<td></td>
<td><input type='radio' name='Keuze' value='Algemene info' checked />Ik wens algemene info</td>
</tr>
<tr>
<td></td>
<td><input type='radio' name='Keuze' value='Andere vraag' />Andere vraag</td>
</tr>

<tr>
<td><b>Naam</b></td>
<td><input name='Naam' type='text' class='textfield' maxlength='60' id='Naam' size='52' value='". $_POST['Naam'] ."' /><font color='red'>*</font></td>
</tr>

<tr>
<td><b>Telefoon</b></td>
<td><input type='text' name='Telefoon' maxlength='60' size='52' value='". $_POST['Telefoon'] ."' /></td>
</tr>

<tr>
<td><b>E-mail</b></td>
<td><input type='text' name='Email' maxlength='60' size='52' value='". $_POST['Email'] ."' /><font color='red'>*</font></td>
</tr>

<tr>
<td><b>Opmerkingen</b></td>
<td><textarea name='opmerkingen' cols='45' rows='6'>". $_POST['opmerkingen'] ."</textarea><font color='red'>*</font></td>
</tr>
<tr>
<td><img src='CaptchaSecurityImages.php' alt='' /> <b>Security Code:</b> </td><td><input id='security_code' name='security_code' type='text' /></td>
</tr>
<tr>
<td> </td>
<td><input type='submit' name='submit' value='Verzenden' /></td>
</tr>
</table>";


I have the issue that when I do a
print "Code: " . $_SESSION['security_code'];
it's not filled up in the external script.

Do you have any idea what I'm doing wrong? I'm a newbie at Modx...

As you also can see I already tried this startCMSSession() :
//require_once ($_SERVER["DOCUMENT_ROOT"] . '/manager/includes/config.inc.php');
//startCMSSession();
But without succes.

Thanks you already,
Filip