<?php

class struct {
    private $args;
    function __construct(array $args) {
        $this->args = $args;
    }
    function __invoke(callable $signature = NULL) : array {
        return $signature(...array_values($this->args));
    }
}

final class Dinosaur extends struct {
    function signature(int $legs) : array {
        return get_defined_vars();
    }
}

$two_legs = function(Dinosaur $dino) : Dinosaur {
    $trex = $dino([$dino, 'signature']);
    $trex['legs'] = 2;
    return new Dinosaur($trex);
};

var_dump($two_legs(new Dinosaur([0])));