Q on safe constants in plugins

I'm working on my first plugins and I need a couple of simple constants, CR (for a '\n') and BR (for '
').

In regular PHP, this is no big deal because I control all my code and so know if I'm using a constant. But because I'm building something that lives in someone else' code (and there are no real namespaces for this sort of thing), I'm wondering how you add constants and know you're not stepping on other code?

Obviously, I could create a constant like RIDICULOUSLY_LONG_SPECIAL_NAME_CR, but I like BR and CR because they're easy to code.

Anyway, enough talk for a simple question. How do you handle constants in your plugins?