A Little History…
Nearly 13 years ago, Michel Valdrighi quietly added some code to his
program which unknowingly was to become one of WordPress’s greatest abilities:
That code commit back in October 2002 contans a single note: “now with b2_filter support !” – perhaps the exclamation point exposing a little thrill that this was something new and exciting, but I have to wonder if he could actually imagine what it really meant for the future.
In fact with the addition of those functions “add_filter/apply_filters” one can begin to easily recognize some of the oldest source code as WordPress itself, compared to earlier versions.