I have a script built in php (which is open source) and i want to heavily modify it (including it's look). I have checked the files and there is no copyright, no license, no legal information...

If i create my new script based on the existing one, will i own the copyright for it? If not, can someone prove that they are the rightful owners of this initial script (if yes, how?)

I didn't find any help related to copyright of code, especially as it's open source code.

Hope anyone can help.