Captcha Code does not work

[frank_solo]

I'm having a problem with captcha code. I keep getting the echo message "CAPTCHA CODE does not match!" everytime I send it . What am I not seeing? Please open my eyes to the error. Thanks in advance.

<?php
session_start();

include "config.php";
$id = (int)$_GET['id'];
$res = mysql_query($query, $bd);
$id = (int)$_GET['id'];
$id = substr($id, 0,5);
if($id < 1 || $id > 99999) exit;
$query = "SELECT * FROM `units` WHERE `id` = $id";
$res = mysql_query($query, $bd);
$result = mysql_fetch_assoc($res);
$contact = $result['contact'];
$phone = $result['phone'];
 
$var = $result['email'];
if (!empty($var)) {

if (isset($_POST['submit'])) {

if(($_SESSION['security_code'] == $_POST['security_code']) && (!empty($_SESSION['security_code'])) ) {
$to       = "$var"; // change to your email address
$name     = htmlentities ($_POST['name']);
$email    = htmlentities ($_POST['email']);
$phone    = htmlentities ($_POST['phone']);
$msg      = htmlentities ($_POST['msg']);
$d        = date('l dS \of F Y h:i:s A');
$sub      = "My Domain RE:".$result['title']."";
$headers  = "From: $name <$email>\n";  
$headers .= "Content-Type: text/plain; charset=iso-8859-1\n";
$mes      = "phone: ".$phone."\n";
$mes     .= "Message: ".$msg."\n";
$mes     .= "Name: ".$name."\n";
$mes     .= 'Email: '.$email."\n";
$mes     .= 'Date & Time: '.$d;
mail($to, $sub, $mes, $headers);

echo "<div align='left'<span style='font-family: Tahoma, Geneva, sans-serif; font-size: 14px; color: #F00;'>Email Has Been Sent</span></div>";
} else {

echo "<div align='left'<span style='font-family: Tahoma, Geneva, sans-serif; font-size: 14px; color: #F00;'>CAPTCHA CODE does not match! </span></div>";

}

} 
echo "<div align='left'><span style='font-family: Tahoma, Geneva, sans-serif; font-size: 14px; color: #00F;'>Email: ".$result['contact']."</span></div>
<form method='post' action='apt.php?id=$id' name='contact''>
<input name='name' type='text' size='40' placeholder='Name'/><br>
<input name='email' type='text' size='40' placeholder='Email'/><br>
<input name='phone' type='text' size='40' placeholder='Phone'/><br>
<textarea name='msg' cols='29' rows='4'></textarea><br>
<div align='left'><img src='http://www.mydomain.co/CaptchaSecurityImages.php?width=100&height=40& characters=5' alt='captcha' width='75px'/><br><input name='security_code' type='text' id='security_code' /><input type='submit' value='Send' class='button' name='submit' /></div>
</form><br><br><br>";}
?>

[mac_gyver]

what have you done to troubleshoot the problem? what does $_SESSION['security_code'] contain?