CakeDC\Users\Model\Behavior\SocialBehavior::generateUniqueUsername PHP Method

    public function generateUniqueUsername($username)
    {
        $i = 0;
        while (true) {
            $existingUsername = $this->_table->find()->where([$this->_table->alias() . '.username' => $username])->count();
            if ($existingUsername > 0) {
                $username = $username . $i;
                $i++;
                continue;
            }
            break;
        }
        return $username;
    }