PHP Constants

PHP constants are name or identifier that can't be changed during the execution of the script. PHP constants can be defined by 2 ways:

1. Using define() function
2. Using const keyword

PHP constants follow the same PHP variable rules. For example, it can be started with letter or underscore only.

Conventionally, PHP constants should be defined in uppercase letters.

PHP constant: define()

Let's see the syntax of define() function in PHP.

define(name, value, case-insensitive)  

1. name: specifies the constant name
2. value: specifies the constant value
3. case-insensitive: Default value is false. It means it is case sensitive by default.

Let's see the example to define PHP constant using define().

File: constant1.php

<?php  
define("MESSAGE","Hello Skyapper PHP");  
echo MESSAGE;  
?>  

Output:

Hello Skyapper PHP

File: constant2.php

<?php  
define("MESSAGE","Hello Skyapper PHP",true);//not case sensitive  
echo MESSAGE;  
echo message;  
?>  

Output:

Hello Skyapper PHPHello Skyapper PHP

File: constant3.php

<?php  
define("MESSAGE","Hello Skyapper PHP",false);//case sensitive  
echo MESSAGE;  
echo message;  
?>  

Output:

Hello Skyapper PHP

Notice: Use of undefined constant message - assumed 'message'

in C:\wamp\www\vconstant3.php on line 4

message

PHP constant: const keyword

The const keyword defines constants at compile time. It is a language construct not a function.

It is bit faster than define().

It is always case sensitive.

File: constant4.php

<?php  
const MESSAGE="Hello const by Skyapper PHP";  
echo MESSAGE;  
?>  

Output:

Hello const by Skyapper PHP