PHP Constants
A constant is an identifier (name) for a simple value. The value cannot be changed during the script. By convention, constant identifiers are always uppercase.
1. Using define()
The define() function allows you to define constants at runtime.
define("GREETING", "Welcome to W3Schools.com!");
echo GREETING;- Scope: Constants defined with
define()are global and can be used anywhere in the script.
2. Using const Keyword
The const keyword allows you to define constants at compile time. This is often used inside classes, but can be used in the global scope as well.
const MY_CAR = "Volvo";
echo MY_CAR;3. define() vs const
| Feature | define() |
const |
|---|---|---|
| Time of Definition | Runtime | Compile time |
| Scope | Global | Global or Class/Namespace |
| Conditionals | Can be used inside if blocks |
Cannot be used inside if blocks |
Example: Conditional Definition
if (!defined('DEBUG')) {
define('DEBUG', true); // Works
}
// if (...) { const DEBUG = true; } // Syntax Error4. Magic Constants
PHP provides a large number of predefined constants to any script which it runs. These change depending on where they are used.
__LINE__: The current line number of the file.__FILE__: The full path and filename of the file.__DIR__: The directory of the file.__FUNCTION__: The function name.__CLASS__: The class name.__METHOD__: The class method name.__NAMESPACE__: The name of the current namespace.