My current web application uses about 30 or so Contants (DEFINE()). I am reading things that variables are quicker. Provided that there is a naming convention to avoid variable overwrites, the only other draw back I see is that these variables would have to be defined as global variables some how in every function.
Which is faster? I use these constants a whole lot throughout my application and will probably be forever adding more to the list and they are used in and out of functions and classes.
Constants defined using define() are fairly slow in PHP. People actually wrote extensions (like hidef) to improve the performance.
But unless you have loads of constants this shouldn't make much of a difference.
As of PHP 5.3 you can also use compile-time constants using const NAME = VALUE;. Those are much faster.