How to: Copyright your PHP software?

I have been googling on this subject but I can't find an answer to the specific questions I have about copyrighting software built on PHP.

I understand that PHP is licensed as open source and any derived works have to be licensed under the same agreement as the original. However, as I understand that is referring to derivatives of the actual PHP source code, and not software/applications coded in PHP.

My question is this: Can I copyright an application I wrote in PHP? ... and if so, is that considered a derived work? Even though I'm using an open source language to create an application, can my application qualify as a non-open source copyright?

Thanks,
Jesse