Function to preserve custom URL parameters on OPC pages after adding a product to the cart

woocommerce_add_to_cart_redirect.php

<?php
/** 
 * Function to preserve custom URL parameters on OPC pages after adding a product to the cart
**/

add_filter( 'woocommerce_add_to_cart_redirect', 'opc_keep_affiliate_arg', 11, 1 );
function opc_keep_affiliate_arg( $url ) {
	if ( ! is_ajax() && is_wcopc_checkout() ) {
		$schema = is_ssl() ? 'https://' : 'http://';
		$url = explode('?', $schema . $_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"] );
		$url = remove_query_arg( array( 'add-to-cart', 'variation_id', 'quantity' ), $url[0] );
		
		//$referral_var = affiliate_wp()->tracking->get_referral_var();
		$referral_var = 'aff';
		$url = add_query_arg( $referral_var, $_GET[$referral_var], $url );
	}
	
	return $url;
}
?>