Licenses, what can and can't I use for my internship?
 

Well... the title says it all really...
I am an intern at a company that specializes in back-ends of websites (a dream internship BTW :)) plus I am also in the process of building my own website.
All of this with PHP and MySQL databases.

Now in my internship I often run into the problem that I want to do something and I know it's already out there, Open Source. But I believe the GPL restricts you from using Open Source code in closed source software. So I have to reinvent the wheel constantly. Fortunately at my current internship I am working on an existing piece of code where things like MySQL database connections, querybuilding, tablebuilding etc. is already dealt with.
But what if my next internship isn't? What if I want to make a website with a decent back-end freelance style to sell?

Should I start building my own PHP library that I can use for work and for my private use, simply because I own the intellectual copyright? Is that even legal? Or desirable for that matter?
Could I make it open source but in such a way that I could still use it in closed source myself and companies I work for?
Does anyone have any experiance or expertise on these matters?

well, your options pretty much consist of
#1: writing your own code
#2: finding a non-gpl solution

gpl is written in such a way that it encourages open-source projects. people wouldn't donate to something that would just turn around and make some corporation money... the gpl is set up so that if you use a gpl component, the result has to be released gpl as well, encouraging open-source contributions. That's not gonna work for you, and you won't be using GPL open-source code at work. There are other free-ware licenses out there, and other places to find software, so don't give up on open-source immeadiately, but well... you're pretty well stuck there.

The GPL allows usage doesn't it?

A little research goes a long way.

http://en.wikipedia.org/wiki/GPL

Yes, you can charge for GPL software, but you must provide access to the source and anyone else can redistribute it at no charge under the same terms, among other restrictions. What you cannot do is take GPL code and redistribute it under an incompatible license (e.g., a license that restricts the rights to modification and distribution) in modified or original form. There is no obligation to distribute your modifications, but if you do, you must abide by the terms of the license.

Which means internal company use is cool.

ok, so you can sell something you make with gpl code... but you have to offer the source for free. If I worked for a software company, think my boss wouldn't be real pleased if i put gpl code in our new flagship application, then had to release the source.

Not all software developed is meant to be sold. GPL code is still very useful, even in a business environment.

Be careful about building your own libraries, by the way. If you build them on company time, they belong to the company. If you build them on your own time, but with company projects in mind, they still arguably belong to the company.

I would be careful placing your personal code library in a program thats closed source and sold if you ever plan on using that exact coding again.

It then belongs to the company you wrote it for.