class Person
{
    #[SerializedName('1')]
    private string $firstname;

    #[SerializedName('2')]
    private string $lastname;

    #[SerializedName('3')]
    private string $email;
   
    /*
         getter and setter
    */   
}

$person = new Person();
$person->setFirstname('John');
$person->setLastname('Doe');
$person->setEmail('john.doe@example.com');

$jsonContent = $this->serializer->serialize($person, 'json');

/*
        {
    	    "1": "John",
    	    "2": "Doe"
    	    "3": "john.doe@example.com"
	}
*/

$deserializePerson = $this->serializer->deserialize($jsonContent, Person::class, 'json');

/*
	App\Dto\Person Object
	(
	    [firstname:App\Dto\Person:private] => Doe
	    [lastname:App\Dto\Person:private] => john.doe@example.com
	)
*/