Is using the directory separator constant necessary? - language-agnostic

Question

I am using PHP, but I guess this question might be language agnostic.
With PHP, a constant is defined by PHP, called DIRECTORY_SEPARATOR. I have seen this in Joomla
define('DS', DIRECTORY_SEPARATOR);
I thought this looked like a good idea so I incorporated it into some of my sites. Now I'm asking myself why. I have only experience on Windows and OS X and from what I know Microsoft, Linux and Apple all use the forward slash as the directory separator.
Is using this constant unnecessary?

Answer 1

As far as PHP is concerned, you might not need it when constructing a path, but it is important for anything you get from the OS.
From http://alanhogan.com/tips/php/directory-separator-not-necessary:
In attempting to write cross-platform, portable PHP code, I used PHP’s DIRECTORY_SEPARATOR constant to write path strings, e.g. "..".DIRECTORY_SEPARATOR."foo", because the “proper” way to do it on Windows would be "..\foo" while on everything else (Linux, UNIX, Mac OS X) it would be "../foo".
Well, as Christian on php.net pointed out and the guys at Web Design Forums confirmed, that’s completely unnecessary. As long as you use the forward slash, “/”, you’ll be OK. Windows doesn’t mind it, and it’s best for *nix operating systems.
(Note that DIRECTORY_SEPARATOR is still useful for things like explode-ing a path that the system gave you. Thanks to Shadowfiend for pointing this out.)

Answer 2

Mac OS Classic uses ":", for instance. See Wikipedia for details. Also it's considered good style avoiding 'magic numbers' or similar constructs.

Answer 3

Windows actually uses a backslash as the directory separator, although some environments that have Windows versions will translate between forward slashes and backslashes automatically (Python comes to mind).

Answer 4

I learned that the best way to write cross-platform compatible code is to never use DIRECTORY_SEPARATOR, or backslashes \\, and to ONLY use forward slashes /.
Why? Because a backslash directory separator ONLY works on Windows. And forward slashes works on Linux, Windows, Mac altogether.
Using the constant DIRECTORY_SEPARATOR or escaping your backslashes \\ can become messy. Look at this example:
$file = 'path' . DIRECTORY_SEPARATOR . 'to' . DIRECTORY_SEPARATOR . 'file';
$file = str_replace('/', DIRECTORY_SEPARATOR, 'path/to/file';
$file = (strtoupper(substr(PHP_OS, 0, 3)) === 'WIN') ? 'path\\to\\file' : 'path/to/file';
When you can just do this:
$file = 'path/to/file';
The only downside I came across is that PHP will return backslashes on Windows for all file references from functions like realpath(), glob(), and magic constants like __FILE__ and __DIR__. So you might need to str_replace() them into forward slashes to keep it consistant.
$dir = str_replace('\\', '/', realpath('../'));
I am hoping that PHP in the future will introduce some php.ini setting to always return forward slashes.