Notes on Object Oriented Programming in PHP - Classes and Class Attributes

Classes, which are the abstracted form of objects, have methods and attributes which in regular programming would be very similar to functions and variables. 

-How Attributes vary from Variables:
They have visibility and must be constants. Yet it is not required that an attribute possess a value to begin with. The key is that attributes can be declared with a variable as the value, it must be either empty (which would be its assigned value) or a pre-defined constant.