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<?php /* readonly */ class ImmutableCounter { public function __construct(private readonly int $count) {} public function add1(): self { return new self($this->count + 1); } public function value(): int { return $this->count; } } // assuming the proposed RFC is in place, this is now possible: class MutableCounter extends ImmutableCounter { public function __construct(private int $count) {} public function add1(): self { return new self(++$this->count); } public function value(): int { return $this->count; } } $counter1 = new ImmutableCounter(0); $counter2 = $counter1->add1(); $counter3 = $counter2->add1(); var_dump([$counter1->value(), $counter2->value(), $counter3->value()]); $mutableCounter1 = new MutableCounter(0); $mutableCounter2 = $mutableCounter1->add1(); $mutableCounter3 = $mutableCounter2->add1(); var_dump([$mutableCounter1->value(), $mutableCounter2->value(), $mutableCounter3->value()]);