lsp.php

<?php

interface Identifiable {

	/**
	 * @return Id
	 */
	public function getId();

	/**
	 * Needs to be an id of the correct type.
	 * If the type is wrong, an InvalidArgumentException is thrown.
	 *
	 * @param Id $id
	 */
	public function setId( $id );

}

class Kitten implements Identifiable {

	/**
	 * @param KittenId $id
	 */
	public function setId( $id ) {
		if ( !( $id instanceof KittenId ) ) {
			throw new InvalidArgumentException( 'Needs to be a KittenId' );
		}
	}

}

class Pony implements Identifiable {

	/**
	 * @param PonyId $id
	 */
	public function setId( $id ) {
		if ( !( $id instanceof PonyId ) ) {
			throw new InvalidArgumentException( 'Needs to be a PonyId' );
		}
	}

}

abstract class Id {
	public abstract function toString();
}

class KittenId extends Id {
	// Makes sure all Kitten ids start with a "k"
}

class PonyId extends Id {
	// Makes sure all pony ids start with a "p"
}