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");