ho i have writen this code for my captcha. i created my captcha for contact form everything works fine but what er the number i entered in captcha box it shows invalid captcha ``

<?php
if(isset($_POST['norobot']))
 {
  if(md5($_POST['norobot']) == $_SESSION['randomnr2'])
  {
  echo "Validation Success";
  $_SESSION['name'] = $name ;
  $_SESSION['phone_no'] = $phone;
  $_SESSION['mailid'] = $mailid;
  $_SESSION['msg'] = $msg;
  $_SESSION['category'] = $category;
  header("Location:thankyou.php");
  }
  else
   {
   $Error = 'Invalid CAPTCHA';
   }
 }
}
?>   

can any one say the solution.

thanks