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
    Create Namespaced Classes with MooTools

    MooTools has always gotten a bit of grief for not inherently using and standardizing namespaced-based JavaScript classes like the Dojo Toolkit does.  Many developers create their classes as globals which is generally frowned up.  I mostly disagree with that stance, but each to their own.  In any event...

  • By
    Convert XML to JSON with JavaScript

    If you follow me on Twitter, you know that I've been working on a super top secret mobile application using Appcelerator Titanium.  The experience has been great:  using JavaScript to create easy to write, easy to test, native mobile apps has been fun.  My...

Incredible Demos

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!