attribute.php

<?php

#[Sealed(ExtendedUserInterface::class, User::class)]
interface UserInterface {}

// ok
interface ExtendedUserInterface extends UserInterface {}

// ok
class User implements UserInterface {}

// error: cannot extend sealed interface UserInterface
interface MyUserInterface extends UserInterface {}

// error cannot implement sealed interface UserInterface
interface Myuser implements UserInterface {}

#[Sealed(Bar::class)]
class Foo {}

// ok
class Bar extends Foo {}

// error: Cannot extend sealed class Foo.
class Baz extends Foo {}

// ok
class Qux extends Bar {}

builtin-attribute.php

<?php

#[Sealed(Exception::class, Error::class)]
interface Throwable {}

#[Sealed(Partial::class)]
interface Closure {}

builtin-keyword.php

<?php

interface Throwable for Exception, Error {}

interface Closure for Partial {}

keyword.php

<?php

interface UserInterface for User, ExtendedUserInterface {}

// ok
interface ExtendedUserInterface extends UserInterface {}

// ok
class User implements UserInterface {}

// error: cannot extend sealed interface UserInterface
interface MyUserInterface extends UserInterface {}

// error cannot implement sealed interface UserInterface
interface Myuser implements UserInterface {}

class Foo for Bar {}

// ok
class Bar extends Foo {}

// error: Cannot extend sealed class Foo.
class Baz extends Foo {}

// ok
class Qux extends Bar {}