add_submenu_page in WordPress

When using the add_submenu_page to create a new submenu in WooCommerce, I get “Cannot modify header information – headers already sent”. The add_submenu_page is as follows:

    public function slp_add_menu_item() {
        add_submenu_page(
            'woocommerce',
            'Shipping Locations',
            'Shipping Locations',
            'manage_options',
            'slp_add_states',
            array( $this->slp_settings_page() ),
        );
    }

Changing that to the following removes the “Cannot modify header information – headers already sent” errors but fails to render callback function properly:

    public function slp_add_menu_item() {
        add_submenu_page(
            'woocommerce',
            'Shipping Locations',
            'Shipping Locations',
            'manage_options',
            'slp_add_states',
            array( $this, 'slp_settings_page' ),
        );
    }

The slp_settings_page callback function, in turn, renders an instance of WP_List_Table (It’s not a taxonomy despite the appearances in the screenshots above). The add_submenu_page is added through the admin_menu hook without any priorities.

Any ideas on what’s happening here? I can’t seem to wrap my head around it 🙁

Answer

The first code block is wrong. According to the documentation the last parameter should be a callable. The first block directly executes the function and stores the result in an array (with the result being null, by the way). So, passing that as a callable won’t work.

Why the second block isn’t working exactly I don’t know. Perhaps the code block gets executed at the wrong time, perhaps something else is amiss.