Use DIRECTORY_SEPARATOR instead of always using forward slash

[mnelson4]

If you try to run WordPress on windows, where the directory separator is a backslash instead of a forward slash, there are a few spots where the folder paths are wrong.
We should always be using forward slashes in URLs, but filepaths should use the DIRECTORY_SEPARATOR constant. This way, the filepaths are ok on Windows machines too.

For example, wp-config-sample.php has this:

<?php
if ( !defined('ABSPATH') )
        define('ABSPATH', dirname(__FILE__) . '/');

Which, in windows, is producing a path like this: 'E:\Software\laragon\www\laragonwp/`. Notice the last slash is the wrong direction! That can cause problem when a Windows machine is trying to read a file in that filepath.

That code should instead be

<?php
if ( !defined('ABSPATH') )
        define('ABSPATH', dirname(__FILE__) . DIRECTORY_SEPARATOR);

I think these filepath issues have gone unnoticed because it's pretty rare to run WordPress directly on a Windows machine (usually folks use a virtual machine or linux subsystem on Windows 10). But I think this might become more prevalent because I'm seeing some better tools for Windows (eg ​https://laragon.org/) and running npm from within a virtual machine using a Windows host has problems (eg https://core.trac.wordpress.org/ticket/44271)

[mnelson4]

Oh, I actually just learned PHP is ok with using forward slashes, even when ran from Windows (see ​https://stackoverflow.com/questions/3554817/using-directory-separator-in-file-exists-on-windows#3554966). My problems were just in unit tests that incorrectly expected the operating system's directory separator to be used.
I ranted on this topic quite a bit on my blog: ​https://cmljnelson.wordpress.com/2018/07/26/which-way-do-your-slashes-face/
But basically, while I'd prefer to use DIRECTORY_SEPARATOR, I think the dye has been cast, WordPress uses forward slashes in PHP code, even for filepaths. So I think it's ok to just close this.

[desrosj]

Thanks @mnelson4!

I looked through Core and the only locations that use DIRECTORY_SEPARATOR currently are things like PHPMailer, ID3m and the random compatibility library. The only exception is the ms-site.php file.

I'm OK closing this to keep consistency, but marking maybelater in case something changes down the road.

[mnelson4]

Thanks for taking a look @desrosj!
Ya I think if this were a real issue someone else would have said so.

[SergeyBiryukov]

Related: #15598, #16457, #17494, #20849, #28811, #29726.