gistfile1.php

<?php

interface Comparable {
    function compare(self $compare);
}

class Foo implements Comparable {
    function compare(self $compare) {}
}

class Bar implements Comparable {
    function compare(self $compare) {}
}

$foo = new Foo();
$bar = new Bar();

$foo->compare($foo);
$bar->compare($bar);

@andriess I think the interface works fine:

http://3v4l.org/VJTnU

<?php

interface Comparable {
function compare(self $compare);
}

class Foo implements Comparable {
function compare(Comparable $compare) {}
}

class Bar implements Comparable {
function compare(Comparable $compare) {}
}

$foo = new Foo();
$bar = new Bar();

$foo->compare($foo);
$bar->compare($bar);

@Ocramius, now the method accepts any class that implements the comparible interface. I want to define that a method can only accept an instance of itself.

@andriesss that could work with the static keyword (unsupported), but is still wrong.

Pretty lame, seems like a nice feature, I guess.

@andriesss the fact is that every implementation basically accepts different types of objects. That's a mess from a LSP pov

@Ocramius, the only option I have is:

<?php

interface Comparable {
function compare(Comparable $compare);
}

class Foo implements Comparable {
    function compare(Comparable $compare) {
        if (!$compare instanceof self) { 
            throw new \InvalidArgumentException('sometimes php sucks a little');
        }
    }
}

class Bar implements Comparable {
    function compare(Comparable $compare) {
        if (!$compare instanceof self) { 
            throw new \InvalidArgumentException('sometimes php sucks a little');
        }
    }
}

$foo = new Foo();
$bar = new Bar();

$foo->compare($foo);
$bar->compare($bar);

@andriesss that's about right... Again, I don't think that's a PHP issue.