PHP Magic Constants

Magic constants are the predefined constants in PHP that get changed on the basis of their use. They start with double underscore (__) and ends with double underscore.

They are similar to other predefined constants but as they change their values with the context, they are called magic constants. Magic constants are case-insensitive.

There are eight magical constants defined in the below table.

Name	Description
__LINE__	Represents current line number where it is used.
__FILE__	Represents full path and file name of the file. If it is used inside an include, name of included file is returned.
__DIR__	Represents full directory path of the file. Equivalent to dirname(__file__). It does not have a trailing slash unless it is a root directory. It also resolves symbolic link.
__FUNCTION__	Represents the function name where it is used. If it is used outside of any function, then it will return blank.
__CLASS__	Represents the function name where it is used. If it is used outside of any function, then it will return blank.
__TRAIT__	Represents the trait name where it is used. If it is used outside of any function, then it will return blank. It includes namespace it was declared in.
__METHOD__	Represents the name of the class method where it is used. The method name is returned as it was declared.
__NAMESPACE__	Represents the name of the current namespace.

Let see the examples one by one.

__LINE__

<?php
echo "Line number " . __LINE__ . "<br>"; // Displays: Line number 2
echo "Line number " . __LINE__ . "<br>"; // Displays: Line number 3
?>

Output:

--------------------

Line number 2
Line number 3

__FILE__

<?php
// Displays the absolute path of this file
echo "The full path of this file is: " . __FILE__;
?>

Output:

------------------------

The full path of this file is: C:/localhost/demo/main.php

__DIR__

<?php
// Displays the directory of this file
echo "The directory of this file is: " . __DIR__;
?>

Output:

------------------

The directory of this file is: C:/localhost/demo/

__FUNCTION__

<?php
function mainFunction(){
    echo  "The function name is - " . __FUNCTION__;
}
mainFunction(); // Displays: The function name is - myFunction
?>

Output:

---------------

The function name is - mainFunction

__CLASS__

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

Output:

----------------

DemoClass

__METHOD__

<?php
class DemoClass
{
    public function mainMethod(){
        echo __METHOD__;
    }
}
$obj = new DemoClass();
$obj->mainMethod(); // Displays: Sample::myMethod
?>

Output:

-------------------

DemoClass::mainMethod

__NAMESPACE__

<?php
namespace PhpNamespace;
class DemoClass
{
    public function getNamespace(){
        return __NAMESPACE__;
    }
}
$obj = new DemoClass();
echo $obj->getNamespace(); // Displays: MyNamespace
?>

Output:

-----------------------

PhpNamespace