Carbon_Fields\Container\Post_Meta_Container::is_valid_attach PHP Method

    public function is_valid_attach()
    {
        global $pagenow;
        if ($pagenow !== 'post.php' && $pagenow !== 'post-new.php') {
            return false;
        }
        // Post types check
        if (!empty($this->settings['post_type'])) {
            $post_type = '';
            if ($this->post_id) {
                $post_type = get_post_type($this->post_id);
            } elseif (!empty($_GET['post_type'])) {
                $post_type = $_GET['post_type'];
            } elseif ($pagenow === 'post-new.php') {
                $post_type = 'post';
            }
            if (!$post_type || !in_array($post_type, $this->settings['post_type'])) {
                return false;
            }
        }
        // Check show on conditions
        foreach ($this->settings['show_on'] as $condition => $value) {
            if (is_null($value)) {
                continue;
            }
            switch ($condition) {
                case 'page_id':
                    if ($value < 1 || $this->post_id != $value) {
                        return false;
                    }
                    break;
                case 'parent_page_id':
                    // Check if such page exists
                    if ($value < 1) {
                        return false;
                    }
                    break;
            }
        }
        return true;
    }