I have the following code:

index.php:
class main_class {

public $database = new DAL;
public $html = new HTML;

}

dal.php:
class DAL {

function method() {
# Code
}

}

class HTML {

function method() {
# Code
# HOW TO CALL DAL'S METHOD?
# Code
}

}

As you see, DAL and HTML are inside two variables of two classes, but
how can they interact with each other?