I know that you can create global constants in terms of each other using string concatenation:
define(\'FOO\', \'foo\'); define(\'BAR\', FOO.\'bar\'); echo
Always fall back to the trusty manual for stuff like this.
Regarding constants:
The value must be a constant expression, not (for example) a variable, a property, a result of a mathematical operation, or a function call.
So... "no" would be the answer :D