Unit_Testing.php

<?php
declare(strict_types=1);

final class username
{
    private $username;

    private function __construct(string $username)
    {
        $this->ensureIsValidUsername($username);

        $this->username = $username;
    }

    public static function fromString(string $username): self
    {
        return new self($username);
    }

    public function __toString(): string
    {
        return $this->username;
    }

    private function ensureIsValidUsername(string $username): void
    {
        if (!filter_var($username, FILTER_VALIDATE_USERNAME)) {
            throw new InvalidArgumentException(
                sprintf(
                    '"%s" is not a valid username',
                    $username
                )
            );
        }
    }
}