Only allow 1 item in WooCommerce Cart

gistfile1.txt

// before add to cart, empty the existing cart items
add_filter( 'woocommerce_add_to_cart_validation', 'sa53re_custom_woocommerce_add_to_cart_validation' );
 
function sa53re_custom_woocommerce_add_to_cart_validation( $cart_item_data ) {
 
    global $woocommerce;
    $woocommerce->cart->empty_cart();
 
    // Do nothing with the data and return
    return true;
}