Simple evaluator

eval.php

<?php
declare(strict_types=1);

interface Expr {
    public function e(Evaluator $eval);
    public function unbox();
}

abstract class LRExpr implements Expr {
    private $l;
    private $r;
    public function __construct(Expr $l, Expr $r) { $this->l = $l; $this->r = $r; }
    public function l(): Expr { return $this->l; }
    public function r(): Expr { return $this->r; }
    public function unbox(): Expr { return $this; }
}

class I implements Expr {
    private $i;
    public function __construct(int $i) { $this->i = $i; }
    public function e(Evaluator $eval): I { return $eval->eI($this); }
    public function unbox(): int { return $this->i; }
}

class B implements Expr {
    private $b;
    public function __construct(bool $b) { $this->b = $b; }
    public function e(Evaluator $eval): B { return $eval->eB($this); }
    public function unbox(): bool { return $this->b; }
}

class Add extends LRExpr {
    public function e(Evaluator $eval): I { return $eval->eAdd($this); }
}

class Mul extends LRExpr {
    public function e(Evaluator $eval): I { return $eval->eMul($this); }
}

class Eq extends LRExpr {
    public function e(Evaluator $eval): B { return $eval->eEq($this); }
}

class Evaluator {
    public function e(Expr $expr): Expr { return $expr->e($this); }
    public function eI(I $i): I { return $i; }
    public function eB(B $b): B { return $b; }
    public function eAdd(Add $a): I { return new I($this->e($a->l())->unbox() + $this->e($a->r())->unbox()); }
    public function eMul(Mul $m): I { return new I($this->e($m->l())->unbox() * $this->e($m->r())->unbox()); }
    public function eEq(Eq $expr): B { return new B($this->e($expr->l())->unbox() === $this->e($expr->r())->unbox()); }
}

$isTrue = (new Evaluator)->e(
    new Eq(
        new Add(new I(4), new I(5)),
        new Mul(new Add(new I(1), new I(2)), new I(3))
    )
);

\assert($isTrue->unbox() === true);