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.
If you have no information, then you can’t know it is open source. BTW open source has a license - so if it is, the rest of your questions can be answered by that.
Alternatively, sounds like you wanna rip someone off 
in which case it would be their proof against yours.
How do you know that the script is open source, if you don’t know the license?
In general, a piece of work (the source code) belongs to the person that produced it. Even if they don’t put a copyright statement in there, it still belongs to them. Proving that in court is a different matter however.
Depends. If you make additions/modifications, then you would own those, but you wouldn’t own the entire product, so they would (legally speaking) be quite useless. If you rewrite all the code, then you’re probably safe, although I’m not a lawyer.
Usually, small cases are never tried in court, so you may not have to worry about any of this. However, if you make money from it, it could become a problem for you. I would try to figure out who made the code and ask them to license it to you, before proceeding.
I am just saying - i will use 30% of the same code, but it will be written by me. My main question (and concern is) how can you define copyright for an expression like <? echo “hello world” ?> ?
Let’s say a script has 3 lines and one of the is the one above. If i create another 3 script line, that does the 99% same thing as the first and it contains the echo above, will it be considered copyright infringement?
For php code for example, how can you prove that you OWN 10 lines of code that make a script? And if you can OWN them, does this mean that no one else is allowed to use them in the same configuration?
Law is not an exact science - It will always be a matter of subjective judgment. So it’s a continuum where on one end you are clearly copying it verbose and on the other end there is no reuse at all. The closer you are to either extreme, the more plausible it is that a judge would rule either way.
In general, I find that code is rarely that unique that you really have to reuse it. If you understand the code, you can usually reimplement it from scratch, and often it will come out better that way. Is there any reason why you can’t simply rewrite everything?
Unless the licence states that you are allowed to create a derivative work then you are not legally allowed to do so.
Where was the code originally obtained from?
Ah… I always think for this type of questions regarding the copyright. That means if rewrite everything with simple changes in the variables and coded in some different ways with the same concept, then there will not be any legal issue? If yes, then no one is safe with their property if someone sells his product once with the source code because anyone (programmer) can rewrite the same code and sells and earns money.
Really worrying issue for the developers 
!!!
It comes down to whether they created a derivitive work by using that code as a starting point or wrote their own version from scratch.
If they started from the existing code and made changes then if they made enough changes they may eventually reach the point where the code cannot be identified as being derived from the original. There would probably be more work involved in doing that than in writing your own version just using the ideas from the original but without actually copying any of it.
Depends on your business model. If you make your money from selling something more than just lines of code, then that doesn’t need to be a problem. For example, if you provide a library, you could sell security patches and new versions as part of the package.