Sets a custom Cancel URL for PayPal transactions

pmpro-set-custom-paypal-cancel-url.php

//From: http://www.paidmembershipspro.com/forums/topic/paypal-express-change-the-cancel-and-return-url/
//Created by user @ultione

add_filter( "pmpro_set_express_checkout_nvpstr", "custom_set_paypal_express_cancel_url", 10, 2 );

function custom_set_paypal_express_cancel_url ( $nvpStr, $order ) {
	parse_str( $nvpStr, $payment_arr );

	$level_id = pmpro_getLevel( 'membership_level_name' );
	if ( ! empty( $level_id ) ) {
		$payment_arr['CANCELURL'] = pmpro_url("checkout", "?level=" . $level_id, "https");
	}
	//To prevent Recurring payment start date to be url encoded.
	$endStr = "&PROFILESTARTDATE={$payment_arr['PROFILESTARTDATE']}";
	unset( $payment_arr['PROFILESTARTDATE'] );

	$nvpStr = '&' . http_build_query( $payment_arr ) . $endStr;
	return $nvpStr;
}