enl

/**
 * This class is purely immutable. 
 * PHP protects the string from changing by its copy-on-change mechanics
 * If you do `$str = $immutable->getField();` and then change `$str`
 * PHP will copy the value and change only second one.
 */
class Immutable 
{
    /** @var string */
    private $field;

enl

class Color {}
class Address {}

class Cat {
    private Int age;
    private Double weight;
    private String name;
    private Address address;
    private Color color;