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Constants are like variables except that once they are defined they cannot be changed or undefined
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).
To create a constant, use the define() function.
Syntax
Parameters:
Create a constant with a case-sensitive name:
<?php
define("GREETING", "Welcome to W3C!");
echo GREETING;
?>
Create a constant with a case-insensitive name:
<?php
define("GREETING", "Welcome to W3C!", true);
echo greeting;
?>
In PHP7, you can create an Array constant using the define() function.
Create an Array constant:
<?php
define("cars", [
"Alfa Romeo",
"BMW",
"Toyota"
]);
echo cars[0];
?>
Constants are automatically global and can be used across the entire script.
This example uses a constant inside a function, even if it is defined outside the function:
<?php
define("GREETING", "Welcome to W3C!");
function myTest() {
echo GREETING;
}
myTest();
?>