Viart Shop

Hi all,
I got this problem I can't figure out:
I am trying to use paypal payment system, in sandbox. after the payment is done on paypal I get this:
Fatal error: Call to undefined function get_param() in /mounted-storage/home81b/sub004/sc44664-SMCA/www/web/shop/payments/paypal_pdt.php on line 20
It seems that the pdt is having problems with the "tx" parameter
Any suggestions on how to fix this?
thanks in advance,
ewoud

My bad, I figured it out...