Differences between constants and variables

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 and invalid constant names: 

// 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");