Solution:
You could use WC()->cart->empty_cart();
instead
function filter_woocommerce_add_to_cart_validation( $passed, $product_id, $quantity, $variation_id = null, $variations = null ) {
// When NOT empty
if ( ! WC()->cart->is_empty() ) {
// Empties the cart and optionally the persistent cart too.
WC()->cart->empty_cart();
}
return $passed;
}
add_filter( 'woocommerce_add_to_cart_validation', 'filter_woocommerce_add_to_cart_validation', 10, 5 );
If the above does not work for some reason, you can also apply it in the following way: (Solution for PHP 7.3 and up)
// Used to calculate totals
function action_woocommerce_before_calculate_totals( $cart ) {
if ( is_admin() && ! defined( 'DOING_AJAX' ) )
return;
if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 )
return;
// Get cart
$get_cart = $cart->get_cart();
// Solution for PHP 7.3 and up
foreach ( $get_cart as $cart_item_key => $cart_item ) {
// NOT last element
if ( $cart_item_key !== array_key_last( $get_cart ) ) {
// Remove a cart item
$cart->remove_cart_item( $cart_item_key );
}
}
}
add_action( 'woocommerce_before_calculate_totals', 'action_woocommerce_before_calculate_totals', 10, 1 );