Read return values from admin-ajax function

Can anyone please guide me to how to get this done.
Well my case is i need to pass some values to my payment gateway and that part i got managed.The next is i need to process the return response from the gateway and show the payment is Ok. So i made a server-side function payment_handler and added it as an ajax running function

add_action( 'wp_ajax_payment_handler', 'payment_handle_callback' );
add_action( 'wp_ajax_nopriv_payment_handlen', 'payment_handle_callback' );

.So my return URL is something like this (https://nibaya.com/ccc/wp-admin/admin-ajax.php?action=paymenthandler&status=success&checkout=true&varable=result….). I think i have a problem in reading this inside my function.I tried without passing any parameters like https://nibaya.com/ccc/wp-admin/admin-ajax.php?action=paymenthandler it works(i put a mail and a jscript alert).But when i add some parameters it gives me 500 error.Here i must have done something wrong here.Any guidance please!