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PHP | Constants

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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 constant
define("WELCOME", "GeeksforGeeks");
echo WELCOME, "\n";
  
// This creates a case-insensitive constant
define("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.


Last Updated : 09 Mar, 2018
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