<?php
declare(strict_types=1);

final class Quantity
{
    public int $units {
        set {
            if ($value < 0) {
                throw new InvalidArgumentException('Units must be non-negative.');
            }
            $this->units = $value;
        }
    }

    public function __construct(int $units)
    {
        $this->units = $units;
    }
}

$quantity = new Quantity(3);
try {
    $quantity->units = -1;
} catch (InvalidArgumentException $error) {
    echo $error->getMessage(), PHP_EOL;
}
echo $quantity->units, PHP_EOL; // 3
try {
    new Quantity(-1);
    throw new RuntimeException('Invalid construction was accepted');
} catch (InvalidArgumentException) {
    echo "Invalid construction rejected", PHP_EOL;
}
$quantity->units = 5;
echo $quantity->units, PHP_EOL;