Stringable

<?php

class FullName {
    private string $lastname;
    private string $firstname;

    public function __construct(string $lastname, string $firstname) {
        $this->lastname = $lastname;
        $this->firstname = $firstname;
    }

    public function __toString() {
        return $this->firstname . " " . $this->lastname;
    }
}

class Hello {
  public static function sayHello(string|Stringable $name): void {
      echo $name;
  }
}

Hello::sayHello(new FullName("Teszt", "Elek"));
Hello::sayHello("Viktor");