尝试而是使用template_redirect操作钩子重新访问了类似的代码：

Try instead this revisited similar code using template_redirect action hook:

add_action( 'template_redirect', 'auto_add_product_to_cart' );
function auto_add_product_to_cart() {
    if ( is_admin() ) return;

    // Product Id of the free product which will get added to cart;
    $product_id = 99999;

    if ( WC()->cart->is_empty() ) 
    {
        // No products in cart, we add it
        WC()->cart->add_to_cart( $product_id ); 
    } 
    else
    {
        $found  = false;

        //check if product already in cart
        foreach ( WC()->cart->get_cart() as $cart_item ) {
            if ( $cart_item['data']->get_id() == $product_id )
                $found = true;
        }

        // if the product is not in cart, we add it
        if ( ! $found )
            WC()->cart->add_to_cart( $product_id ); 
    }
}

代码进入function.php文件您的活动子主题（或活动主题）。经过测试并有效。

注意： init 钩在这里是不正确的，因此不能用于此……

Note: The init hook is not correct here and not to be used for this…