Differences between constants and variables are?
There is no need to write a dollar sign ($) before a constant, where as in Variable one has to write a dollar sign.
Constants cannot be defined by simple assignment, they may only be defined using the define() function.
Constants may be defined and accessed anywhere without regard to variable scoping rules.
Once the Constants have been set, may not be redefined or undefined.
// Valid constant names
define("ONE", "first thing");
define("TWO2", "second thing");
define("THREE_3", "third thing");
// Invalid constant names
define("2TWO", "second thing");
define("__THREE__", "third value");