function_exists works. If it says it's there, it's there. As I see in gd.c file, these functions are always defined, but not always would work. If you don't like it, submit change request.

the functions are there even if the
format is not supported just to
give an error message to tell you
that the feature is not supported

this is in there because otherwise
the "undefined function" error
messages would confuse users

use function imagetypes to test
for available formats