Laravel Create Unique Slug in AppServiceProvider

AppServiceProvider.php

    public function boot()
    {
        Post::saving(function ($post) {
            $title = $post->title;
            $proposed_slug = strtolower(str_replace(' ', '-', $title));

            $existing_slug = Post::where('slug', $proposed_slug)->where('id', '<>', $post->id)->first();

            if(! $existing_slug) {
                $post->slug = $proposed_slug;
            } else {
                $number_added_to_proposed_slug = 0;
                do {
                    $number_added_to_proposed_slug += 1;
                    $numbered_proposed_slug = $proposed_slug . '-' . $number_added_to_proposed_slug;
                    $existing_slug = Post::where('slug', $numbered_proposed_slug)->Where('id', '<>', $post->id)->first();
                } while ($existing_slug);
                $post->slug = $numbered_proposed_slug;
            }
        });
    }