Now someone gets a copy of my a.php and your b.php and runs a.php . In order to execute it, his system copies a.php into memory (this is a copy of my a.php) and then processes it to include b.php inside. That processed copy is derived from both a.php and b.php .
BTW: I really like your idea about adaptations. So if I have a printer whose drivers don't allow me to do silly things, is it within my right to adapt them? Distribute the resulting code? For profit?