Identifiers are used for define names, variable names, array names, and function names.
Identifiers must start with an alphabetic character or underscore ('_') character, and may be following be any number of alphanumeric characters or underscore.
These are all valid identifiers:
Indentifiers are not case sensitive.
For example, 'Test', 'TEST' and 'test' are all the same identifiers.
However, it is allowed for identifiers to be reused for functions.