I'm still learning about domain driven design, and I have created my first entity class User.Is this fully compliant with the DDD principles?  Particularly my setters because I have business rules in them.  Any other suggestions would be highly appreciated as well.namespace Models\Entities;class User{    private $id;    private $firstName;    private $lastName;    private $email;    private $password;    private $registerDate;    public function setId($id)    {        if (!$this->id) {            if (is_int($id) && $id >= 1) {                $this->id = $id;                return $this;            }            throw new \InvalidArgumentException('ID must be a positive integer.');        }        throw new \InvalidArgumentException('ID has already been set.');    }    public function getId()    {        return $this->id;    }    public function setFirstName($firstName)    {        if (is_string($firstName) && !empty($firstName) && ctype_alpha($firstName)) {            $length = $this->stringLength($firstName);            if ($length >= 1 && $length <= 35) {                $this->firstName = $firstName;                return $this;            }        }        throw new \InvalidArgumentException('First name must be an alpha string inclusively between 1 and 35 characters.');    }    public function getFirstName()    {        return $this->firstName;    }    public function setLastName($lastName)    {        if (is_string($lastName) && !empty($lastName) && ctype_alpha($lastName)) {            $length = $this->stringLength($lastName);            if ($length >= 1 && $length <= 35) {                $this->lastName = $lastName;                return $this;            }        }        throw new \InvalidArgumentException('Last name must be an alpha string inclusively between 1 and 35 characters.');    }    public function getLastName()    {        return $this->lastName;    }    public function getFullName()    {        return $this->firstName . ' ' . $this->lastName;    }    public function setEmail($email)    {        if (is_string($email) && !empty($email)) {            $sanitized = filter_var($email, FILTER_SANITIZE_EMAIL);            $isValid   = filter_var($sanitized, FILTER_VALIDATE_EMAIL);            $isValid   = $isValid ? $this->stringLength($email) <= 254 : false;            if ($email === $sanitized && $isValid) {                $this->email = $email;                return $this;            }        }        throw new \InvalidArgumentException('Email must be a string in this format: [local]@[domain].[ext].');    }    public function getEmail()    {        return $this->email;    }    public function setPassword($password)    {        if (is_string($password) && !empty($password)) {            $length = $this->stringLength($password);            if ($length >= 6 && $length <= 60) {                $this->password = $password;                return $this;            }        }        throw new \InvalidArgumentException('Password must be a string inclusively between 6 and 60 characters.');    }    public function getPassword()    {        return $this->password;    }    public function setRegisterDate($date)    {        if (is_int($date) && !empty($date)) {            // TO DO: date validation unix timestamp is_int            $this->registerDate = $date;            return $this;        }        throw new \InvalidArgumentException('Register date must be an integer and valid unix timestamp.');    }    public function getRegisterDate()    {        return $this->registerDate;    }    private function stringLength($input)    {        $encoding = mb_detect_encoding($input);        $length   = mb_strlen($input, $encoding);        return $length;    }}
First domain-driven User entity class
php;object oriented;ddd
Your class can absolutely be considered like a DDD domain entity.However, I would suggest some improvements:DDD likes to use an ObjectValue for the unique identifiers, you could use one instead of a simple integer.You could separate the validation logic from the entity (not specific to DDD).You should not have a getPassword() method but a verifyPassword($password) instead. Don't expose the password in plain text.You could also separate the stringLength() into its own string utility class.Regarding the use of a Uniw timestamp, keep in mind that DateTime is a better option since it embeds the time zone. Timestamps don't, which is always confusing when handling dates from all over the world...