[development] implementing "plugins"

[Dan Robinson]

Hi,

I want to introduce a concept of "plugins" to my module.  In this case
the plugin merely needs to supply a single function that returns data. 
Is the best way to do this to have the plugin call a register function
with the name of a function (and then call "eval"?).  This is probably
more of a PHP question than a Drupal question.  Oh yes and is there
documentation somewhere on what code Drupal automatically "includes"?

Thanks,

Dan