|Don't ask to ask, just ask|
So why we can't change $c=1000?
As roboticus points out, you can make that change. That is, you can overwrite the value in $c -- currently the address of the constant 99 -- with any other value.
What you cannot do is overwrite the value held at the address held in $c, because it is a constant.
With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.