PHP constants

PHP Constants

• A constant is an identifier (name) for a simple value. The constant value cannot be 

changed during the script. 

• By default a constant is case-sensitive.

• By convention, constant identifiers are always uppercase. 

• A valid constant name starts with a letter or underscore (no $ sign before the 

constant name). 

• If you have defined a constant, it can never be changed or undefined. 

• Note: Unlike variables, constants are automatically global across the entire script. 

Set a PHP Constant 

• To set a constant, use the define() function – 

• It takes three parameters: The first parameter defines the name of the constant, the 

second parameter defines the value of the constant, and the optional third parameter 

specifies whether the constant name should be case-insensitive. 

• Default is false. 

• The example below creates a case-sensitive constant, with the value of "hello world". 

Example: 

<?php 

 define("GREETING", "hello!"); 

 echo GREETING; 

 ?> 

constant() function 

• As indicated by the name, this function will return the value of the constant. 

• This is useful when you want to retrieve value of a constant, but you do not know its 

name, i.e. It is stored in a variable or returned by a function. 

<?php 

define("MINSIZE", 50); 

echo MINSIZE; 

echo constant("MINSIZE"); // same thing as the previous line 

?> 

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.