PHP - Captcha, problème de sessions

Bonjour,

J'ai un soucis. J'utilise PHP à l'occasion mais là c'est la première fois que j'utilise les sessions dans mes formulaires. J'ai crée une image captcha mais je n'arrive pas à récupérer le code captcha pour le vérifier...

Voici la structure:

dossier "pages"
fichier "contact.php"

<a href="index.php?pageid=contact" target="_top">:: Contact ::</a>

<p>
<style type="text/css">
form {
	text-align:left;
	margin-left: 40px;
}
</style>
<form action="contact_vf.php" method="post" enctype="multipart/form-data" name="form1">
  <p>
    <label for="name"><b>Name/Nickname*</b></label><br />
    <input name="name" type="text" class="input" id="name" size="25" maxlength="20">
  </p>
  <p>
    <label for="email"><b>Email*</b></label><br />
    <input name="email" type="text" class="input" id="email" size="30" maxlength="90">
  </p>
  <p>
    <?php echo "<input name=\"ip\" type=\"hidden\" id=\"ip\" value=\"".$_SERVER['REMOTE_ADDR']."\">"; ?>
  </p>
  <p>
    <label for="website"><b>Website</b></label><br />
    <input name="website" type="text" class="input" id="website" size="35" maxlength="200">
  </p>
  <p>
    <label for="subject"><b>Subject*</b></label><br />
    <input name="subject" type="text" class="input" id="subject" size="45" maxlength="20">
  </p>
   <p>
	<label for="message"><b>Message*</b></label><br />
	<textarea name="message" id="message" cols="100" rows="16" class="input"></textarea>
  </p>
  <p>
	<label for="captcha"><b>Captcha*</b></label><br />
	<table width="128px" border="0">
	<tr>
    <td><img src="captcha.php" width="54" height="16" name="captcha" id="captcha"/><?php echo $_SESSION['captcha_text']; ?></td>
    <td><input name="captcha_text" type="captcha_text" class="input" id="captcha_text" size="10" maxlength="6" /></td>
	</tr>
	</table>
  </p>
  <p><br />
    <input name="submit" type="submit" class="button" id="submit" value="Submit">
	<p style="text-align: right;"><b>*Required fields.</b></p>
  </p>
</form>

</p>

dossier principal:
"captcha.php"

<?php
session_start();
$max = 6;
$captcha_text = strtoupper(substr(md5(rand()),0,$max)); 
$_SESSION['captcha_text'] = $captcha_text;

$captcha = imagecreate(54,16);
$red = imagecolorallocate($captcha, 255, 0, 0);
$BGRand = imagecolorallocate($captcha, mt_rand(0,63), mt_rand(0,63), mt_rand(0,63));
$COLORand = imagecolorallocate($captcha, mt_rand(180,255), mt_rand(180,255), mt_rand(180,255));
$LINERand = imagecolorallocate($captcha, mt_rand(0,31), mt_rand(0,31), mt_rand(0,31));

for($i = 0; $i < 30; $i++)
{
ImageLine($captcha,mt_rand(0,54),mt_rand(0,16),mt_rand(0,54),mt_rand(0,16),$BGRand);
}
imagestring($captcha, 5, 0, 0, $captcha_text, $COLORand);

for($i = 0; $i < 3; $i++)
{
ImageLine($captcha,mt_rand(0,54),mt_rand(0,16),mt_rand(0,54),mt_rand(0,16),$LINERand);
}

header ("Content-type: image/png");
header("Cache-Control: no-store, no-cache, must-revalidate");
imagecolortransparent($captcha, $red);
imagepng($captcha);
?>

"contact_vf.php" (C'est juste pour tester que tout fonctionne bien, après je vais la modifier pour reçevoir le message par mail)

<?php
function CheckMail($mail) 
{ 
   if(preg_match('#^[\w.-]+@[\w.-]+\.[a-zA-Z]{2,6}$#',$mail)) 
        return true; 
   else 
        return false; 
}

// if (isset($_POST['name']) && isset($_POST['email']) && isset($_POST['ip']) && isset($_POST['subject']) && isset($_POST['message']) && isset($_SESSION['captcha_text']) && isset($_POST['captcha_txt']) && $_POST['captcha_txt'] != "" && $_SESSION['captcha_text'] != "" && $_POST['name'] != "" && $_POST['email'] != "" && $_POST['ip'] != "" && $_POST['subject'] != "" && $_POST['message'] != "" && Checkmail($_POST['email']))
    // {
        $name = htmlspecialchars($_POST['name']);
        $email = htmlspecialchars($_POST['email']);
        $ip = htmlspecialchars($_POST['ip']);
        $subject = htmlspecialchars($_POST['subject']);
        $message = htmlspecialchars($_POST['message']);
        $captchaORIGINAL = htmlspecialchars($_SESSION['captcha_text']);
        $captchaUSER = htmlspecialchars($_POST['captcha_text']);
    // }
        echo "NAME:".$name."<br>";
        echo "EMAIL:".$email."<br>";
        echo "IP:".$ip."<br>";
        echo "SUBJECT:".$subject."<br>";
        echo "MSG:".$message."<br>";
        echo "Ocaptcha:".$captchaORIGINAL."<br>";
        echo "Ucaptcha:".$captchaUSER."<br>";
?>

Merci