Domain-Driven Design in PHP

Domain Services

Throughout conversations with Domain Experts, you'll come across concepts in the Ubiquitous Language that can't be neatly represented as either an Entity or a Value Object, such as:

• Users being able to sign into systems by themselves
• A shopping cart being able to become an order by itself

The preceding example are two concrete concepts, neither of which can naturally be bound to either an Entity or a Value Object. Further highlighting this oddity, we can attempt to model the behavior as follows:

class User{    public function signUp($aUsername, $aPassword)    {        // ...    }}class Cart{    public function createOrder()    {        // ...    }}

In the case of the first implementation, we're not able to know that the given username and password relate ...