PHP error - use of undefined constant at line x... what does this mean?

This error means exactly that - you are using a constant that has not been declared anywhere in the script.

Now, if you mean to be using constants in the script, then you simply have forgotten to define it, so make sure that you do so.

More often than not however, this happens when you forget to put the $ infront of a variable name, and so you just need to insert the $ immediately before the name of that variable to fix the error.