Adding custom macros into Laravel

AppServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Support\ServiceProvider;
use Illuminate\Support\Collection;

class AppServiceProvider extends ServiceProvider
{
    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        Collection::macro('ifEmpty', function (callable $callback): Collection {
            if ($this->isEmpty()) {
                $callback($this);
            }
            return $this; 
            /* what does $this mean in this scope?
               collect()->ifEmpty(function($e) {error_log('empty '.get_class($e));})
               empty Illuminate\Support\Collection
            */
        });
    }

    /**
     * Register any application services.
     *
     * @return void
     */
    public function register()
    {
        //
    }
}