Use this class to practise TDD and refactoring the code

Legacy 01

<?php
class User
{
    private $userId;
    private $firstName;
    private $lastName;
    private $email;
    private $password;
    
    private $salt;
    
    public function __construct(array $options)
    {
        foreach ($options as $key => $value) {
            if (property_exists($this, $key)) {
                $this->{$key} = $value;
            }
        }
    }

    public function createPassword()
    {
        $this->salt = substr(str_shuffle("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"), 0, 15);
    
        $this->password = sha1($this->password . $this->salt);
    }

    public function verifyPassword($password)
    {
        return ($this->password === sha1($password . $this->salt));
    }

    public function isInputValid()
    {
        if (empty($this->firstName) || empty($this->lastName) || empty($this->email) || empty($this->password) || ! filter_var($this->email, FILTER_VALIDATE_EMAIL)) {
            return false;
        }
    
        return true;
    }
    
    private function sendActivationEmail()
    {
        global $config;
        
        $email = new \Util\Mail($config);
        
        $email->setEmailFrom($config->email);
        
        $email->setEmailTo($this->email);
        
        $email->setTitle('Your account has been activated');
        
        $email->setBody("Dear {$this->firstName}\n Your account has been activated\n");
        
        $email->send();
    }
    
    public function createUser()
    {
        global $config;
        
        if (!$this->isInputValid()) {
            return false;
        }
        
        $this->createPassword();
        
        $db = $config->db;
        
        /* @var $db \PDO */
        $sql = "INSERT INTO users(firstname) VALUES (:firstname)";
        
        $statement = $db->prepare($sql);
        
        $statement->bindParam(':firstname', $this->firstName);
        
        if ($statement->execute()) {
            $this->userId = $db->lastInsertId();
        
            $this->sendActivationEmail();
    
            return true;
        } else {
            throw new \Exception(implode(':', $statement->errorInfo()));
        }
    }
}