Immutable

An immutable object is an object that can’t be changed. It may be created, read and cloned. On the other hand, a mutable object may change when one of its method is called, even if this is not visible from the outside.

Immutable objects are thread-safe.

Immutable objects often return a clone version of themselves, after modification.

<?php

class x {
     private $x = 0;

     function inc() : self {
             $return = clone $this;
             $return->x +=1;

             return $return;
     }
}

$a = new x();
$b = $a->inc();

// $a and $b are two distinct objects

?>

Documentation

See also https://dev.to/timoschinkel/the-case-for-immutability-1gfa, https://dev.to/hbgl/elegant-immutable-object-pattern-in-php-1dg3, https://medium.com/@mohamadshahkhajeh/immutable-objects-in-php-your-secret-weapon-for-safer-applications-04eb88c49150