Why would you want to make constants out of post data?

I'm wondering whether making constants out of post data will be safe and good practice to do.

It's safe. is the same method as $ _POST, it just replaces the names of variables by constants. This class has the same function as $ _POST. Only simplifies how to use them.

It could cause conflict if you post a form field with a name of an existing constant. Another possible hiccup would be that when a constant value CONSTANT_NAME isn't defined it will not default to NULL, but PHP will assume you meant a string of "CONSTANT_NAME"; though this shouldn't be an issue with proper validation.

In a multi-tier application, you'd really be mucking about with mixing user-input and business logic (you have this problem with a $_POST superglobal as well, but atleast it's easily identifiable as a superglobal). Constants should be reserved for constant variables. User input by definition isn't constant; if the user doesn't make a submission, then the constant won't exist. Not really "wrong" in the sense that it won't work, but wrong in the sense that you're using a screw driver to hammer a nail.

Thanks for your reply @Kyle Barrett. That was exactly the concern I had.
Using constants in a place where a variables should be used will probably complicate things rather than simplifying.
One can argue that a user input remains constant as far as the duration of a single request-response cycle is concerned. But even that will not convince me it is a right way to deal with user data. The $_POST array keeps the post data completely isolated from other data in your program.
I feel that creating constants out of post could be compared to emptying a waste bin all over the floor.

I fully agree with your answer @Kyle, this class can be used for something more simple as display data or simply make a check if there are any empty field without getting making "empty" all the time. In one of my scripts, I found some problems when using constants for inserting in the database, so I use this class to do only checks if the field is empty.
Well, now each one will adapt this script to your needs.