PHP invariant types. Test script.

test_script.php

<?php

// example 1: return type
class A
{
    function foo(): A
    {
        return $this;
    }
}

class B extends A
{
    function foo(): B
    {
        return $this;
    }
}
// error: PHP Fatal error:  Declaration of B::foo(): B must be compatible with A::foo(): A in ...


// example 2: arguments type hinting
class X {}
class Y extends X {}

class A
{
    function foo(X $bar) {}
}
class B extends A
{
    function foo(Y $bar) {}
}
// error: PHP Warning:  Declaration of B::foo(Y $bar) should be compatible with A::foo(X $bar) in ...

@IMSoP nice graphics :)

With LSP in mind of course you're right.