Lifetime

I'm curious to know what the lifetime of a constant is. I know that sounds like a stupid question, but the context is this: I have a file I am writing to up to the very last possible micro-second. As such, I know that as PHP is destroying itself after having executed the last statement in the program, things begin to disappear. The Class objects destruct in no discernable order (they have destructors). Any open file handles close. Etc. My question is: During all this destruction, at what point must I no longer expect to have a constant available? That is, when the classes are executing their __destruct() functions, et al, I want to manage something with those destructors. So when does the constant actually-factually disappear?