Extending interfaces

FooImplementation.php

<?php

declare(strict_types = 1);

class FooImplementation implements FooInterface
{
    // Note: this throws an error in PHP 7.1:
    // Declaration of FooImplementation::doSomething(): FooImplementation must be compatible with FooInterface::doSomething(): FooInterface
    public function doSomething(): FooImplementation
    {
        return $this;
    }
}

FooInterface.php

<?php

declare(strict_types = 1);

interface FooInterface
{
    public function doSomething(): FooInterface;
}