PHP | Constants - GeeksforGeeks

09 Mar, 2018
Constants are either identifiers or simple names that can be assigned any fixed values. They are similar to a variable except that they can never be changed. They remain constant throughout the program and cannot be altered during execution. Once a constant is defined, it cannot be undefined or redefined. Constant identifiers should be written in upper case following the convention. By default, a constant is always case-sensitive, unless mentioned. A constant name must never start with a number. It always starts with a letter or underscores, followed by letter, numbers or underscore. It should not contain any special characters except underscore, as mentioned.
Creating a PHP Constant
The define() function in PHP is used to create a constant as shown below:Syntax:
define(name, value, case_insensitive)
The parameters are as follows:
name: The name of the constant.
value: The value to be stored in the constant.
case_insensitive: Defines whether a constant is case insensitive. By default this value is False, i.e., case sensitive.
Example:
<?php  // This creates a case-sensitive constantdefine("WELCOME", "GeeksforGeeks");echo WELCOME, "\n";  // This creates a case-insensitive constantdefine("HELLO", "GeeksforGeeks", true);echo hello;  ?>
Output:
GeeksforGeeks
GeeksforGeeks

constant() function
Instead of using the echo statement ,there is an another way to print constants using the constant() function.
Syntax
constant(name)
Example:
<?php     define("WELCOME", "GeeksforGeeks!!!");        echo WELCOME, "\n";     echo constant("WELCOME");    // same as previous  ?>
Output:
GeeksforGeeks!!!
GeeksforGeeks!!!

Constants are Global: By default, constants are automatically global, and can be used throughout the script, accessible inside and outside of any function.Example:
<?php  define("WELCOME", "GeeksforGeeks");  function testGlobal() {    echo WELCOME;}   testGlobal();  ?>
GeeksforGeeks

Constants vs Variables
A constant, once defined can never be undefined but a variable can be easily undefined.
There is no need to use dollar sign($) before constants during assignment but while declaring variables we use a dollar sign.
A constant can only be defined using a define() function and not by any simple assignment.
Constants dont need to follow any variable scoping rules and can be defined anywhere.
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