<?php

class Immutable
{
    private $a;
    private $b;
    private $c;

    public function __construct($a, $b, $c) {
        $this->a = $a;
        $this->b = $b;
        $this->c = $c;
    }

    //get methods
    //...
    public function __toString() { return "{$this->a} : {$this->b} : {$this->c}"; }

    public function prototype(): ImmutablePrototype {
        return new ImmutablePrototype($this->a, $this->b, $this->c);
    }
}

class ImmutablePrototype
{
    private $a;
    private $b;
    private $c;

    //some data structure would be more common
    public function __construct($a, $b, $c) {
        $this->a = $a;
        $this->b = $b;
        $this->c = $c;
    }

    //set methods
    public function a(string $value): Immutable { $this->a = $a; return $this; }
    public function b(string $value): Immutable { $this->b = $b; return $this; }
    public function c(string $value): Immutable { $this->c = $c; return $this; }

    //validation might happen here if it's costly
    //late failure makes debugging harder, but you don't often pass builder around
    public function build(): Immutable {
        return new Immutable($this->a, $this->b, $this->c);
    }
}