PHP | Defining Constants

Difficulty Level : Basic
Last Updated : 03 Jan, 2018

In a production level code it is very important to keep information as either variables or constants rather than using them explicitly. A PHP constant is nothing but an identifier for a simple value that tends not to change over time(such as the domain name of a website eg. www.geeksforgeeks.org). It is ideal to keep all the constants in a single PHP script so the maintenance is made easier. A valid constant name must start with an alphabet or underscore and requires no ‘$’. It is to be noted, constants are irrespective of their scope i.e constants are automatically of global scope.

In order to create a constant in PHP, we must use the define() method.

Syntax:

bool define(identifier, value, case-insensitivity)

Parameters: The function has two required parameters and one optional parameter.

identifier: Specifies the name to be assigned to the constant.
value: Specifies the value to be assigned to the constant.
case-insensitivity(Optional): Specifies whether the constant identifier should be case-insensitive. By default it is set to false i.e. case-sensitive.

Return Type: This method returns TRUE on success and FALSE on Failure.

Below are some examples to illustrate working of define() function:

Below program illustrates defining case-insensitive constants:

<?php



  // case-insensitive costants

  define("Constant","Hello Geeks!",TRUE);

  echo constant;

  echo Constant;

?>
Output :
```
Hello Geeks!  // Case Insensitive thus value is echoed
Hello Geeks!
```
Below program illustrates defining case-sensitive constants:

<?php



  // case-sensitive constant

  define("Constant","Hello Geeks!");

  echo constant;

  echo Constant;

?>
Output :
```
constant   // Case Sensitive thus value not echoed
Hello Geeks! 
```
The PHP compiler will also throw a warning for above program along with the output as: “PHP Notice: Use of undefined constant constant- assumed ‘constant’ in line 5”.

Summary:

Constants are identifiers that can be assigned values(string, boolean, array, integer, float or NULL) which generally don’t change overtime.
Constants are irrespective of scope and always populate the global scope.
define() method is used to define constants.
defined() method is used to check if a constant is defined.
constant() method is used to return the value of a constant and NULL if not the constant is not defined.

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