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:
Example
<!DOCTYPE html>
<html>
<body>
<?php
// case-sensitive constant name
define(“GREETING”, “Welcome to robertkaramagi.wordpress.com!”);
echo GREETING;
?>
</body>
</html>
Output
The example below creates a constant with a case-insensitive name:
Example
<!DOCTYPE html>
<html>
<body>
<?php
// case-insensitive constant name
define(“GREETING”, “Welcome to robertkaramagi.wordpress.com!”, true);
echo greeting;
?>
</body>
</html>
Output
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:
Example
<!DOCTYPE html>
<html>
<body>
<?php
define(“GREETING”, “Welcome to robertkaramagi.wordpress.com!”);
function myTest() {
echo GREETING;
}
myTest();
?>
</body>
</html>
Output
Robert Karamagi 