ctrlpanel/database/migrations/2022_06_02_081655_referral_code.php

<?php

use App\Models\User;
use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\DB;
use Illuminate\Support\Facades\Schema;
use Illuminate\Support\Str;

class ReferralCode extends Migration
{
    public function generateCode($userid)
    {
        $random = STR::random(8);
        if (User::where('referral_code', '=', $random)->doesntExist()) {
            DB::table('users')
                    ->where('id', '=', $userid)
                    ->update(['referral_code' => $random]);
        } else {
            $this->generateCode($userid);
        }
    }

    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::table('users', function (Blueprint $table) {
            $table->string('referral_code')->lenght(8)->nullable();
        });

        $existing_user = User::where('referral_code', '')->orWhere('referral_code', null)->get();

        foreach ($existing_user as $user) {
            $this->generateCode($user->id);
        }
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::table('users', function (Blueprint $table) {
            $table->dropColumn('referral_code');
        });
    }
}