PHP’s Magic Constants: __LINE__ , __FILE__, __FUNCTION__, __CLASS__, and __METHOD__

By  on  

Setting constants in PHP is as easy as using the define function, but PHP creates a few constants in every script for you that help mostly for debugging purposes (well, that's generally the only time I use them). These constants are called "magic constants."

Magic constants have a funny syntax, placing two underscores before and after the constant's word representation. These are PHP's magic constants:

__LINE__
__FILE__
__FUNCTION__
__CLASS__
__METHOD__

You can imagine how helpful these constants are for debugging...but if you can't:

if($sugar == '') { echo('$sugar has no value on line '.__LINE__.' of ['.__FILE__.']'); }

There are also helpful uses for these magic constants:

dirname(__FILE__) // get the directory name of the current script

What do you use them for?

Recent Features

  • By
    fetch API

    One of the worst kept secrets about AJAX on the web is that the underlying API for it, XMLHttpRequest, wasn't really made for what we've been using it for.  We've done well to create elegant APIs around XHR but we know we can do better.  Our effort to...

  • By
    Camera and Video Control with HTML5

    Client-side APIs on mobile and desktop devices are quickly providing the same APIs.  Of course our mobile devices got access to some of these APIs first, but those APIs are slowly making their way to the desktop.  One of those APIs is the getUserMedia API...

Incredible Demos

  • By
    DWRequest: MooTools 1.2 AJAX Listener & Message Display

    Though MooTools 1.2 is in its second beta stage, its basic syntax and theory changes have been hashed out. The JavaScript library continues to improve and become more flexible. Fellow DZone Zone Leader Boyan Kostadinov wrote a very useful article detailing how you can add a...

  • By
    Build a Calendar Using PHP, XHTML, and CSS

    One of the website features my customers love to provider their web users is an online dynamic calendar. An online calendar can be used for events, upcoming product specials, memos, and anything else you can think of. I've taken some time to completely...

Discussion

  1. My two favorite are

    That you can use it to get parent directory:

    dirname(dirname(__FILE__))
    

    or you can include the init.php file from anywhere in your filesystem:

    You would use

    include_once(dirname(__FILE__) . '/database.class.php');
    

    in the init.php

  2. Wow, awesome usage of the __FILE__ magic constant — I hadn’t thought of that one!

  3. Jaime

    Useful!

  4. To Jesus DeLaTorre:

    “dirname(dirname(__FILE__)”

    Maybe it could be better, when you use this:

    $dir = str_replace ( “\”, “/”, dirname ( _FILE_ ) )
    -> for Windows based systems

    (PS: nice book engine)

  5. thanks for this, i knew only __FILE__ so i googled this article :)

  6. Jim

    @jeezaa:

    Instead of / for linux and \ for windows, just use another constant:

    include_once(dirname(__FILE__) . DIRECTORY_SEPARATOR . 'database.class.php);
  7. Petah

    omg @ the comments

    realpath(dirname(__FILE__)) or

    __DIR__ or

    realpath(dirname(__FILE__).’/..’).’/file.bla’

  8. is there any way to get the constants defined in a php file using
    define()?

  9. I have never seen the use of __FUNCTION__
    Can give me any Example.

  10. Allan Peda

    I make it a habit to wrap these magic constants into SQL and PL/SQL blocks so that I can quickly trace the construction of queries and DML to the source.

  11. There’s also the __NAMESPACE__ constant, e.g.

    namespace hyperdrive;
    add_action('wp_head', __NAMESPACE__ .'\engage');
    

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!