My proposal is the following:
some functions when fed with constant arguments always return a constant
value, too. e.g.:
strlen('abcd') === 4.
I like the general idea.
Would there be some caveats with stuff like this if it is possible to
change the charset at runtime?
I guess it is important to be aware of whether a function is affected
by different settings (and if these settings can be changed at
runtime) to conclude if a function really is deterministic at this
level.
uhm, damn, right. strlen() wasn't a good example.. my bad, sorry :S
Anyway, you got the idea :)
Nuno