How to correctly fork an open-source library? [closed]

Question

I'd like to fork an open-source php-library.

It has its own license, in which is written:

You are permitted to use, copy, modify, and distribute the Software and its documentation, with or without modification, for any purpose, provided that the following conditions are met:

And there are some conditions about providing copy of original license agreement, adding copyrights in every source file, etc.

I want to add new features in this library, which are written under GPL. Then the whole new product should be under GPL? So I should add both GPL and 'old' license agreemenets? And in every source file I should keep both license copyrights?

Answer 1

What was the original license agreement? Your sentencing makes it confusing about whether the original part was GPL or you want to add GPL to it.

If the original license was GPL, then your new software must also be GPL. There is no way around it unless you get the permission from the author or all authors - if there is more than one. You can still sell your product if it is under GPL, but note that the buyer may 'resell' it with whatever price they find appropriate, including free, as long as license conditions are met. GPL is not a problem when building a website or software that is specific to a client, as long as you are fine with giving the client the rights to modify and republish the software.

But if you want to add GPL stuff to non-GPL project, then consider using LGPL license instead. LGPL allows to release the component itself under a GPL-like license while not requiring the other software to be GPL or LGPL in return.