<?php
interface Comparable {
    function compareTo($other): int;
}

class BigInteger implements Comparable {
    public function __construct(private int $value) {}

    public function compareTo($other): int {
        $type = gettype($other);

        switch ($type) {
            case integer::class:
                return $this->compareToInt($other);
            default:
                throw new \LogicException('Cannot compare \''.BigInteger::class.'\' with \''.$type.'\'');
        }
    }

    protected function compareToInt(int $other): int {
        return $this->value - $other;
    }
}

$a = new BigInteger(15);
var_dump($a->compareTo(35));