age.php

<?php

class User {
    
    /**
     * @var DateTimeInterface
     */
    private $birthday;
    
    public function setBirthday(DateTimeInterface $birthday) {
        $this->birthday = $birthday;
    }

    public function getBirthday(): DateTimeInterface {
        if(!($this->birthday instanceof DateTimeInterface)) {
            throw new RuntimeException('Birthday not set');
        }
        return $this->birthday;
    }

}

$user = new User();
# $user->setBirthday(new DateTime('1980-01-01'));
var_dump($user->getBirthday());