Here's the concept of what I mean by a "MySQL Callback Function":
Let's suppose I have a php object OBJ_1 which contains data extracted from rows N_1 and M_1 of tables TBL_A and TBL_B, respectively.
One of the purposes of OBJ is to provide a layer of abstraction between the data and the database - so that when, for example, the database changes, only OBJ needs to change and not the whole of the rest of the code, which makes maintenance of the code SO much easier than having mysql statements sprinkled throughout the code! (which I would call a complete mess).
Anyway, as I have things set up at the moment, every time a new instance of OBJ is created (let's call it OBJ_X), it needs to extract rows N_X and M_X from TBL_A and TBL_B, respectively. This is necessary, because the data in rows N_X and M_X may change at any time (e.g. they may be updated by a separate, asynchronos process).
So, is there a mechanism in standard MySQL for attaching a callback function to rows N_X and M_X of TBL_A and TBL_B, from within a running PHP process? The purpose of the callback function would be to notify OBJ_X that the data in the database has changed in rows N_X and M_X.