if you could undefine, it wouldn't be constant.

I have to admit that the above comment comes off as an
insulting wisecrack. like im a moron for suggesting such
a feature. what would K&R say to that?

Anyway - i found a way around this PHP deficiency for now,
and i did that by creating a set of functions which maintain
an array of defined variables and respond accordinly to my
own undefine, ifdefine, constant and define functions.

cd /web/src/php4; grep -nir "undef" *;

hmm, looks like the php developers found it useful in C

You're on the wrong track comparing C #defines and define() in PHP. C is about compiling and running executables whereas PHP is about interpreting.