PHP Constants

About PHP constents

A constant is an identifier (name) for a simple value. As the name suggests, that value cannot change during the execution of the script (except for magic constants, which aren't actually constants).

Constants are case-sensitive. By convention, constant identifiers are always uppercase. The name of a constant follows the same rules as any label in PHP.

A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.

Use of define() function

Use the define() function to create a constant. It defines constant at run time. Let's see the syntax of define() function in PHP.

Syntax

define(name, value, case-insensitive)

Use of const keyword

PHP introduced a keyword const to create a constant. The const keyword defines constants at compile time. It is a language construct, not a function. The constant defined using const keyword are case-sensitive.

example:

Constant vs Variables in PHP programming

1.) Once the constant is defined, it can never be redefined. A variable can be undefined as well as redefined easily.

2.) A constant can only be defined using define() function. It cannot be defined by any simple assignment. A variable can be defined by simple assignment (=) operator.

3.) There is no need to use the dollar ($) sign before constant during the assignment. To declare a variable, always use the dollar ($) sign before the variable.

4.) Constants do not follow any variable scoping rules, and they can be defined and accessed anywhere. Variables can be declared anywhere in the program, but they follow variable scoping rules.

Constants are the variables whose values can't be changed throughout the program. The value of the variable can be changed.