The opening sentence for the Global Variables chapter reads as follows:

While some global variables can be created through the use of
define(), some are reserved because of a special function, giving access to
different types of data.

Technically, the define() function creates a constant that is available globally, as is described properly as part of the function on page 11. Therefore, the sentence would be better phrased as follows:

While some constants can be made available globally through the use of
define(), some are reserved because of a special function, giving access to
different types of data.