PHP : Form example which will encrypt query string (GET) (data hiding rather than security)

The following solution is easy enough to implement and is strong enough unless you deal with very sensitive data such as credit-cards information or NASA algorithms...

When you send the parameter via. GET - add a hash value along with it, for example:

$parameter = "abc"; //The parameter which you'll pass as a GET parameter
$salt = "cV0puOlx";
$hashed = md5($salt.$parameter);//A hash that you'll pass as well
header("Location: http://www.yourdomain.com?param=$parameter&hash=$hash");

Then when you read the parameters, check that the hash is a valid one:

$parameter  = $_GET['param'];
$hash = $_GET['hash'];
$salt = "cV0puOlx";
$hashed = md5($salt.$parameter);
//now you check:
if ($hash === $hashed){
   //everything's fine - continue processing
}
else{
  // ERROR - the user tried to tamper with your parameter
  // show error-message and bail-out
}