why interfaces ?

all, in recent weeks there has been a lot of arguing about what interfaces are, arent (on php-general and now on the internals list as well) etc. etc. i am quite curious, why interfaces were added to the language in the first place. note, i am not criticizing them, i am overjoyed that the language has them, however i am curious about the history. is this something php borrowed from java? does it have anything to do with 'entirely virtual classes' in c++ ? i think that the history behind the addition of interfaces to php, would shed a lot of light on their intended purpose for use by php developers. thanks, -nathan