__FUNCTION__ returns only the name of the function whereas __METHOD__ returns the name of the class along with the name of the function:

<?php

class trick
{
    public function doit()
    {
        echo __FUNCTION__;
    }

    public function doitagain()
    {
        echo __METHOD__;
    }
}

$obj = new trick();
$obj->doit(); // Outputs: doit
$obj->doitagain();  // Outputs: trick::doitagain

__CLASS__ magic constant returns the same result as get_class() function called without parameters and they both return the name of the class where it was defined (i.e. where you wrote the function call/constant name ).

In contrast, get_class($this) and get_called_class() functions call, will both return the name of the actual class which was instantiated:

<?php

class Definition_Class {

  public function say(){
     echo '__CLASS__ value: ' . __CLASS__ . "\n";
     echo 'get_called_class() value: ' . get_called_class() . "\n";
     echo 'get_class($this) value: ' . get_class($this) . "\n";
     echo 'get_class() value: ' . get_class() . "\n";
  }
  
}

class Actual_Class extends Definition_Class {}

$c = new Actual_Class();
$c->say();
// Output:
// __CLASS__ value: Definition_Class
// get_called_class() value: Actual_Class
// get_class($this) value: Actual_Class
// get_class() value: Definition_Class

Current file

You can get the name of the current PHP file (with the absolute path) using the __FILE__ magic constant. This is most often used as a logging/debugging technique.

echo "We are in the file:" , __FILE__ , "\n";

Current directory

To get the absolute path to the directory where the current file is located use the __DIR__ magic constant.

echo "Our script is located in the:" , __DIR__ , "\n";

To get the absolute path to the directory where the current file is located, use dirname(__FILE__).

echo "Our script is located in the:" , dirname(__FILE__) , "\n";

Getting current directory is often used by PHP frameworks to set a base directory:

// index.php of the framework

define(BASEDIR, __DIR__); // using magic constant to define normal constant

// somefile.php looks for views:

$view = 'page';
$viewFile = BASEDIR . '/views/' . $view;

Separators

Windows system perfectly understands the `/` in paths so the `DIRECTORY_SEPARATOR` is used mainly when parsing paths.

Besides magic constants PHP also adds some fixed constants for working with paths:

$view = 'page';
$viewFile = BASEDIR . DIRECTORY_SEPARATOR .'views' . DIRECTORY_SEPARATOR . $view;

• Rarely used PATH_SEPARATOR constant for separating paths in the $PATH environment variable. It is ; on Windows, : otherwise

Remarks

Magic constants are distinguished by their __CONSTANTNAME__ form.

There are currently eight magical constants that change depending on where they are used. For example, the value of __LINE__depends on the line that it's used on in your script.

These special constants are case-insensitive and are as follows:

Most common use case for these constants is debugging and logging