线程视图中的分层注释系统：PHP和MySQL

I have the need to render a threaded view of a 2-levels hierarchical commenting system. The comments are stored in a database. The information about the hierarchy is given by field 'parent_id' (which is 0 for the top-level comments). I cannot change the structure of the database. The present solution is by means of multiple SQL queries: I have the need to render a threaded view of a