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<?php declare(strict_types = 1); class CustomDateTimeImmutable extends DateTimeImmutable { public function test(): self { return CustomDateTimeImmutable::createFromInterface(new DateTime()); } public function fromFormat(): CustomDateTimeImmutable|false { return CustomDateTimeImmutable::createFromFormat('H:i:s', '00:00:00'); } } var_dump(get_class((new CustomDateTimeImmutable())->test())); //string(23) "CustomDateTimeImmutable" var_dump(get_class((new CustomDateTimeImmutable())->fromFormat())); //string(23) "CustomDateTimeImmutable"