Constants are PHP containers for values that remain constant and never change. They’re mostly used for data that is known well in advance and that is used, unchanged, in multiple places within your application. Good candidates for constants are debug and levels, version numbers, configuration flags, and formula.
Constants are defined using PHP’s define() function, which accept two arguments:
The name of the constant and its value. Constant names must follow the same rule as variable names with one exception: the $ prefix is not required for constant names.
Here’s an example of defining and using a constant in a script:
define(‘PROGRAM’, ‘The Matrix’);
// use constants
//output: ‘Welcome to The Matrix(version 11.7)’
echo ‘Welcome to ‘. PROGRAM. ‘(version’. VERSION. ‘)’;
By convention, constant names are usually entirely uppercased; this is to enable their easy identification and differentiation from “regular” variables in a script.