lsp-violation.php

<?php
class IntCollection {
    private $items = [];
    function set(int $a) {
        $this->items[] = $a;
    }
    function sum(): int {
        return array_sum($this->items);
    }
}

// Does this class violate LSP?
class PositiveIntCollection extends IntCollection {
    function set(int $a) {
        if ($a <= 0) {
            throw new OutOfBoundsException('Only positives bitch!');
        }
        parent::set($a);
    }
}