[php] Why would I use dirname(__FILE__) in an include or include_once statement?

I have seen this:

<?php
  include( dirname(__FILE__) . DIRECTORY_SEPARATOR . 'my_file.php');
?>

Why would I ever need to do this? Why would I go to the trouble of getting the dirname and then concatenating that with a directory separator, and a new filename?

Is the code above not equivalent to this:

<?php
  include( 'my_file.php' );
?>

??

The PHP doc says,

Files are included based on the file path given or, if none is given, the include_path specified. If the file isn't found in the include_path, include() will finally check in the calling script's own directory and the current working directory before failing. The include() construct will emit a warning if it cannot find a file; this is different behavior from require(), which will emit a fatal error.

If you want code is running on multiple servers with different environments,then we have need to use dirname(FILE) in an include or include_once statement. reason is follows. 1. Do not give absolute path to include files on your server. 2. Dynamically calculate the full path like absolute path.

Use a combination of dirname(FILE) and subsequent calls to itself until you reach to the home of your '/myfile.php'. Then attach this variable that contains the path to your included files.

I might have even a simpler explanation to this question compared to the accepted answer so I'm going to give it a go: Assume this is the structure of the files and directories of a project:

Project root directory:
                       file1.php
                       file3.php
                       dir1/
                            file2.php

(dir1 is a directory and file2.php is inside it)

And this is the content of each of the three files above:

//file1.php:
<?php include "dir1/file2.php"

//file2.php:
<?php include "../file3.php"

//file3.php:
<?php echo "Hello, Test!";

Now run file1.php and try to guess what should happen. You might expect to see "Hello, Test!", however, it won't be shown! What you'll get instead will be an error indicating that the file you have requested(file3.php) does not exist!

The reason is that, inside file1.php when you include file2.php, the content of it is getting copied and then pasted back directly into file1.php which is inside the root directory, thus this part "../file3.php" runs from the root directory and thus goes one directory up the root! (and obviously it won't find the file3.php).

Now, what should we do ?!

Relative paths of course have the problem above, so we have to use absolute paths. However, absolute paths have also one problem. If you (for example) copy the root folder (containing your whole project) and paste it in anywhere else on your computer, the paths will be invalid from that point on! And that'll be a REAL MESS!

So we kind of need paths that are both absolute and dynamic(Each file dynamically finds the absolute path of itself wherever we place it)!

The way we do that is by getting help from PHP, and dirname() is the function to go for, which gives the absolute path to the directory in which a file exists in. And each file name could also be easily accessed using the __FILE__ constant. So dirname(__FILE__) would easily give you the absolute (while dynamic!) path to the file we're typing in the above code. Now move your whole project to a new place, or even a new system, and tada! it works!

So now if we turn the project above to this:

//file1.php:
<?php include(dirname(__FILE__)."/dir1/file2.php");

//file2.php:
<?php include(dirname(__FILE__)."/../file3.php");

//file3.php:
<?php echo "Hello, Test!";

if you run it, you'll see the almighty Hello, Test!! (hopefully, if you've not done anything else wrong).

It's also worth mentioning that from PHP5, a nicer way(with regards to readability and preventing eye boilage!) has been provided by PHP as well which is the constant __DIR__ which does exactly the same thing as dirname(__FILE__)!

Hope that helps.

I used this below if this is what you are thinking. It it worked well for me.

<?php
    include $_SERVER['DOCUMENT_ROOT']."/head_lib.php";
?>

What I was trying to do was pulla file called /head_lib.php from the root folder. It would not pull anything to build the webpage. The header, footer and other key features in sub directories would never show up. Until I did above it worked like a champ.