Php instance property mutation

test.php

<?php

class State {
  public $foo = 0;
}

class Mutator {
  public function mutateState(State $state) {
    $state->foo = 4;
  }
}

$state = new State();
echo $state->foo . "\n";

$state->foo = 2;
echo $state->foo . "\n";

$mutator = new Mutator();
$mutator->mutateState($state);
echo $state->foo . "\n";