Can Open Source software be used to develop progams that will be sold?

It depends on the open source license. There are some where it’s OK to do this, and there are others where being open is “viral”… All derivitive works become open as well. You’ll have to read the license of the product you want to modify.

you can not, but you can affix your version name on it. that’s why it is called open source. you have all the rights to improve, modify BUT you need also your newly “programmed” open source to be modified by others…
you name will appear in the list of the modifiers
open source aimed to perfect it’s programmed likewise a ground for developers to share ideas and technology
not like the corporate windows and other sodtware companies

No you can’t you got to suffix the freeware version @ the end of it and you can be in serious trouble in sued…

WordPress is released under the GPL [1], so if you extend it, and include it in a product that is redistributed, then you’d have to include the source code with that product.

It’s a requirement of the GPL that your code that extends the original code must also be made public and released under the GPL so that others could extend it.

Since WordPress is written in PHP, which is an interpreted language; distributing the source code is a technical and not just a legal requirement.

You can, however, provide a web based service that is based on WordPress, or any open source GPL software, and not be required to distribute any source code. That’s totally above board and normal.

Finally, it’s worth noting that WordPress has been developed from a design that is now several years old, so if you’re going to develop something, you might want to reconsider your starting point. WordPress lead developer Matt Mullenweg [2] recently mentioned that if “we could re-write WP from scratch, it would be a lot like bbPress [4]“.