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<?php // note that copy() is a proper alternative to __clone() interface Any { function getHash(); function equals(); } class Mutable implements Any { protected $baz; function __construct(string $baz){ $this->setBaz($baz); } function setBaz(string $baz){ $this->baz = $baz; } function getBaz(){ return $this->baz; } function getHash(){ return Immutable::CLASS . "|" . spl_object_hash($this); } function equals($other){ return $this->getHash() === $other->getHash(); } function copy(){ return new static($this->baz); } } class Immutable implements Any { protected $baz; function __construct(string $baz){ $this->baz = $baz; } function getHash(){ return Mutable::CLASS . "|" . $this->baz; } function equals($other){ return $this->getHash() === $other->getHash(); } function copy(){ return $this; } } var_dump((new Immutable("baz"))->equals(new Immutable("baz"))); // true var_dump((new Immutable("bar"))->equals(new Immutable("bar"))); // true var_dump((new Mutable("baz"))->equals(new Mutable("baz"))); // false var_dump((new Mutable("bar"))->equals(new Mutable("bar"))); // false