liskov.php

<?php

class FooParent
{
}

class Foo extends FooParent
{
}

class FooChild extends Foo
{
}

class Bar
{
    public function func(Foo $test): Foo
    {
        // implementation
    }
}

class BarBadChild extends Bar
{
    public function func(FooChild $test, int $yetAnotherBadArgument): FooParent
    {
        // implementation
    }
}

class BarGoodChild extends Bar
{
    public function func(FooParent $test, int $yetAnotherGoodArgument = null): FooChild
    {
        // implementation
    }
}