Init.php is here to help instanciating and launching all what we need for our modules.
NOTA ! When a class should be instanciated, we call its create_instance method instead of manipulating its construct.
the construct can be used for other things and it permit to call methods that do not depends the current instance object.
in session class for exemple, the garbage collector work on all possible session and not just the current instance.
other example : in DB class create instance create the mysql connection with config param, but the construct can be used to create another connection
on the contrary, HTML_class the html_group_class force the creation of another html due to this instance confusion.