How to make explicit variable type declaration in PHP?

Question

Taking a course on the Internet, I saw the instructor saying that from PHP 4 (he used 3, so did not test) it was possible to make the explicit declaration of type in object attributes as follows: [escopo] $var:[tipo].

Examples:

class ClassePrincipal{
    // como eu quero realizar as conversões
    private $attr:ClasseSecundaria;
    public  $attr:Carbon;

    // como eu faço atualmente para manter uma tipagem forte
    public function exemplo(Obj1 $o1, string $s, int $i):bool{
        return true;
    }
}

But I find nothing on the subject, nor can I carry out such definitions. I know you can do without, but I like to work with strong typing in other languages, and this functionality caught my attention. Is there a way to accomplish them, in the same way that is done with the functions? Already, thank you.