Constants are like variables except that once they are defined they cannot be changed or undefined.

PHP Constants

A constant is an identifier (name) for a simple value. The value cannot be changed during the script.
A valid constant name starts with a letter or underscore (no $ sign before the constant name).
Note: Unlike variables, constants are automatically global across the entire script.

Create a PHP Constant

To create a constant, use the define() function.

Syntax
define(name, value, case-insensitive)
Parameters:
  • name: Specifies the name of the constant
  • value: Specifies the value of the constant
  • case-insensitive: Specifies whether the constant name should be case-insensitive. Default is false
The example below creates a constant with a case-sensitive name:

<!DOCTYPE html>
<html>
<body>

<?php
// case-sensitive constant namedefine("GREETING", "Welcome to AAM Foundation!");
echo GREETING;
?> 

</body>
</html>
The example below creates a constant with a case-insensitive name:


<!DOCTYPE html>
<html>
<body>

<?php
// case-insensitive constant namedefine("GREETING", "Welcome to AAM Foundation!", true);
echo greeting;
?> 

</body>
</html>

Constants are Global

Constants are automatically global and can be used across the entire script.
The example below uses a constant inside a function, even if it is defined outside the function:

<!DOCTYPE html>
<html>
<body>

<?php
define("GREETING", "Welcome to AAM Foundation!");

function myTest() {
    echo GREETING;
}
 
myTest();
?> 

</body>
</html>