---
title: Constants
---
## Constants
Constants are a type of variable in PHP. The `define()` function to set a constant takes three arguments - the key name, the key's value, and a Boolean (true or false) which determines whether the key's name is case-insensitive (false by default). A constant's value cannot be altered once it is set. It is used for values which rarely change (for example a database password OR api key).
### Scope
It is important to know that unlike variables, constants ALWAYS have a global scope and can be accessed from any function in the script.
### Example
```PHP
php.net constants manual
* php.net define() manual
* Create your first PHP class