Abstract.php

<?php

interface ProviderInterface
{
    public function foo();
}

abstract class AbstractProvider implements ProviderInterface
{
    public function bar()
    {
        return "bar\n";
    }

    // foo() is not implemented here
}

class MyProvider extends AbstractProvider
{
    public function foo()
    {
        return "foo\n";
    }
}

$provider = new MyProvider;
echo $provider->foo(); // foo
echo $provider->bar(); // bar

I totaly agree :)

I do prefer having the interface defined in the concrete implementation

This implementation gives flexibility :)

If I want to drop the abstract class for some reasons, I can

Thanks for this explanation.