nagoyaphp21 https://github.com/ttskch/nagoyaphp21/

Nagoya.php

<?php

declare(strict_types=1);

namespace Nagoya;

interface Expression {
    public function toString(): string;
}
class Number implements Expression {
    public function __construct(
        private int $value
    )
    {
    }

    public function toString(): string
    {
        return sprintf('%d', $this->value);
    }
}

enum Operater: string {

    case Addition = '+';
    case Subtraction = '-';
    case Multiplication = '*';
    case Division = '/';

}

class Statement implements Expression {
    public function __construct(
        private Expression $a,
        private Operater $op,
        private Expression $b
    )
    {
    }

    public function toString(): string
    {
        return sprintf('(%s %s %s)', $this->a->toString(), $this->op->value, $this->b->toString());
    }
}

NagoyaTest.php

<?php

declare(strict_types=1);

use Nagoya\Number;
use Nagoya\Operater;
use Nagoya\Statement;
use PHPUnit\Framework\TestCase;

require_once('Nagoya.php');

final class NagoyaTest extends TestCase
{
  public function test2たす3(): void
  {
    $n1 = new Number(2);
    $n2 = new Number(3);
    $exp = new Statement($n1, Operater::Addition, $n2);
    $this->assertSame('(2 + 3)', $exp->toString());
  }
  public function test2たす3ひく4(): void
  {
    $n1 = new Number(2);
    $n2 = new Number(3);
    $n3 = new Number(4);

    $exp = new Statement($n1, Operater::Addition, $n2);
    $exp2 = new Statement($exp, Operater::Subtraction, $n3);
    $this->assertSame('((2 + 3) - 4)', $exp2->toString());
  }
}