Defining and using global variables and global functions, when the convention dictates OOP can be confusing and difficult to use properly for multiple reasons:

* You run the risk of name clashes. 
* Global functions must be stateless, or they can cause difficult-to-track bugs. 
* Global variables can be updated from anywhere and may no longer hold the value you expect.
* It is difficult to properly test classes that use global functions. 

Instead of being declared globally, such variables and functions should be moved into a class, potentially marked ``static``, so they can be used without a class instance. 

This rule  checks that only object-oriented programming is used and that no functions or procedures are declared outside of a class.
