Access private attributes via a php loop

Question

I am making a class for my database interaction and need to assemble my DML commands based on a past class instance.

    foreach($obj as $key => $value){
        //monta comando DML 
    }

This only works if the class attributes are public, in the case of private this foreach does not give any loop. Does anyone have any idea how I can do that? Or the only way would be to oblige the passed object to have a method that returns its own attributes?