Constants and Magic Constants

Last Updated : 23 Jul, 2025

In PHP, constants and magic constants are special identifiers that hold fixed values throughout script execution. They improve code clarity, reduce repetition, and help in debugging or configuration.

This guide covers everything you need to know, from defining constants to exploring the power of magic constants for debugging and performance.

PHP Constants

PHP constants are like variables but with one key difference: their values cannot be changed once they are defined. They are used to store values that should remain consistent throughout the script, making your code more predictable.

Defining PHP Constants

You can define constants in PHP using two methods:

1. Using the define() function:

PHP 
<?php
define("PI", 3.14159);
echo PI;  // Outputs: 3.14159
?>

Output
3.14159

2. Using the const keyword (used for class constants):

PHP 
<?php
const PI = 3.14159;
?>

Class Constants

Constants can also be defined in classes using the const keyword:

PHP 
<?php
class Circle {
    const PI = 3.14159;
}
?>

Output
For more details read the article - PHP Constants
PHP Magic Constants
Magic constants are predefined constants in PHP that change their value based on where they are used in the script. They help with debugging and logging information during script execution.
1. __LINE__ 
This constant returns the current line number in the file where it is used.

            PHP

    
<?php
   echo __LINE__;
?>




Output
2
2. __FILE__ 
This constant gives the full path and name of the file being executed.

            PHP

    
<?php
    echo __FILE__;
?>




Output
/home/guest/sandbox/Solution.php
3. __DIR__ 
Similar to __FILE__, but it returns the directory of the file being executed (without the filename).

            PHP

    
<?php
    echo __DIR__;
?>




Output
/home/guest/sandbox
4. __FUNCTION__ 
This constant returns the name of the function in which it is used.

            PHP

    
<?php
 function myFunction() {
    echo __FUNCTION__;
}
myFunction();
?>




Output
myFunction
5. __CLASS__
 This constant gives the name of the class where it is used.

            PHP

    
<?php
    class MyClass {
    public function getClassName() {
        echo __CLASS__;
    }
}
$obj = new MyClass();
$obj->getClassName();
?>




Output
MyClass
6. __METHOD__ 
This constant returns the name of the method where it is used.

            PHP

    
<?php
 class MyClass {
    public function myMethod() {
        echo __METHOD__;
    }
}
$obj = new MyClass();
$obj->myMethod();
?>




Output
MyClass::myMethod
7. __NAMESPACE__ 
This constant provides the current namespace.

            PHP

    
<?php
    namespace MyNamespace;
echo __NAMESPACE__;
?>




Output
MyNamespace
For more details read the article - PHP  Magic Constants
Differences Between PHP Constants and Magic Constants
PHP constants and magic constants serve different purposes in PHP programming. While both provide fixed values, their behavior and usage vary significantly.
  • Immutability: Constants cannot be changed after definition, whereas magic constants adjust dynamically based on context.
  • Resolution Time: Regular constants are evaluated at runtime, while magic constants are determined at compile time.
  • Usage Context: Constants are user-defined, while magic constants are built into PHP and change depending on where they are used.
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