testInterfaceAdherence.php

<?php

class Tester {
	
	function testUser(User $user): void {
		$user->returnsUserInterface();
		$user->returnsUser();
	}

	function testUserInterface(UserInterface $user): void {
		$user->returnsUserInterface();
		$user->returnsUser(); // to me, this should error-out
	}
	
}

class Factory {
	
	public function getUserInterface(): UserInterface {
		return new User();
	}
	
}

class User implements UserInterface {
	public function returnsUserInterface(): UserInterface {
		echo 'Returning as UserInterface' . PHP_EOL;
		return $this;
	}

	public function returnsUser(): User {
		echo 'Returning as User' . PHP_EOL;
		return $this;
	}
}

interface UserInterface {
	public function returnsUserInterface(): UserInterface;
}


$tester = new Tester();
$factory = new Factory();
$sut = $factory->getUserInterface();

$tester->testUser($sut);
echo PHP_EOL;
$tester->testUserInterface($sut);