Unique initials from first name & last name in Laravel Model Factory

UserFactory.php

<?php

/** @var \Illuminate\Database\Eloquent\Factory $factory */
use App\User;
use Illuminate\Support\Str;
use Faker\Generator as Faker;

/*
|--------------------------------------------------------------------------
| Model Factories
|--------------------------------------------------------------------------
|
| This directory should contain each of the model factory definitions for
| your application. Factories provide a convenient way to generate new
| model instances for testing / seeding your application's database.
|
*/

$factory->define(User::class, function (Faker $faker) {
    static $existingInitials;

    $existingInitials = $existingInitials ?: User::pluck('initials')->toArray();

    $first_name = $faker->firstName;
    $last_name = $faker->lastName;

    $suffix = 0;
    do {
        $safeLastName = Str::ascii($last_name);
        $initials = strtoupper(Str::ascii($first_name)[0]);
        for ($i = 0; $i <= $suffix; $i++) {
            if (! isset($safeLastName[$i])) {
                $initials .= 'X';
                continue;
            }

            $character = $safeLastName[$i];
            if (in_array($character, [' ', '-', '\'', ''])) {
                continue;
            }
            $initials .= strtoupper($character);
        }
        $suffix++;
    } while (in_array($initials, $existingInitials));

    $existingInitials[] = $initials;

    return [
        'initials' => $initials,
        'first_name' => $first_name,
        'last_name' => $last_name,
        'email' => $faker->unique()->safeEmail,
        'password' => '$2y$10$92IXUNpkjO0rOQ5byMi.Ye4oKoEa3Ro9llC/.og/at2.uheWG/igi', // password
        'remember_token' => Str::random(10),
    ];
});