Domain-Driven Design in PHP

Design Aggregates Based in Business True Invariants

First of all, what's an invariant? An invariant is a rule that must be true and consistent during code execution. For example, a stack is a LIFO (Last In, First Out) data structure that we can push items into and pop items out of. We can also ask how many items are inside of the stack; this is what's called the size of the stack. Consider a pure PHP implementation without using any specific PHP array functions such as array_pop:

class Stack {       private $data;    public function __construct()    {        $this->data = [];    }    public function push($value)    {        $this->data[] = $value;    }    public function size()    {        $size = 0;        for ($i = 0; $i < count($this->data); $i++) {            $size++;        }        return $size;    }    /** * @return ...