LinkStack/app/Providers/AppServiceProvider.php

<?php

namespace App\Providers;

use Illuminate\Support\Facades\Validator;
use Illuminate\Support\ServiceProvider;
use Illuminate\Pagination\Paginator;
use Illuminate\Support\Facades\DB;

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

    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        Paginator::useBootstrap();
        Validator::extend('isunique', function ($attribute, $value, $parameters, $validator) {
            $value = strtolower($value);
            $query = DB::table($parameters[0])->whereRaw("LOWER({$attribute}) = ?", [$value]);

            if (isset($parameters[1])) {
                $query->where($parameters[1], '!=', $parameters[2]);
            }

            return $query->count() === 0;
        });
    }
}