What should I use instead of WP_CONTENT_DIR and WP_PLUGIN_DIR?

The WordPress documentation for Determining Plugin and Content Directories states that:

WordPress makes use of the following constants when determining the
path to the content and plugin directories. These should not be used
directly by plugins or themes, but are listed here for completeness.

It goes on to list WP_CONTENT_DIR and WP_PLUGIN_DIR among constants that theme and plugin developers should not use, presumably because of this:

WordPress allows users to place their wp-content directory anywhere
they want, so you must never assume that plugins will be in
wp-content/plugins, or that uploads will be in wp-content/uploads, or
that themes will be in wp-content/themes.

Mark Jaquith also comments here that those constants should not be used:

Don’t use WP_PLUGIN_URL or WP_PLUGIN_DIR — plugins might not be in the
plugins directory.

So, what is the accepted way of referencing the full path to the plugins, wp-content, and themes folders without using these constants?

As a simple example, to output the full path of all installed plugins, I can do this:

<?php
$plugins = get_plugins();

foreach ($plugins as $file => $details) {
    echo WP_PLUGIN_DIR . "https://wordpress.stackexchange.com/" . $file . '<br>';
}

Which produces a list like so:

/var/www/wp-content/plugins/akismet/akismet.php
/var/www/wp-content/plugins/debug-bar/debug-bar.php
/var/www/wp-content/plugins/hello.php

(I might want to do this if I was writing a plugin to allow the user to selectively archive plugins as part of a site backup, for example.)

If using WP_PLUGIN_DIR is wrong, what is the suggested alternative? There is no equivalent to wp_upload_dir() for the plugins, themes, and wp-content folder that I can find, which makes referencing the potentially wandering themes and plugins root directories problematic.

Reference file in current path or deeper nested

To reference the current path

plugin_dir_path( __FILE__ )."further/nesting/here.css";

which works in Plugins and Themes.

Reference URl/URi in a plugin

To point to a plugin or theme file, use

plugins_url( "path/to/file", __FILE__ );

which works only in plugins

Reference URl/URi in wp-admin folder

Always point them to admin_url( 'some/path' );. There is get_admin_url() as well.

Reference URl/Uri in wp-includes folder

Point them to includes_url( 'some/path' );

An URl/URi relative to the sites home

There’s home_url( 'etc' ); or get_home_url() for that. Similar is get_site_url() and site_url(). Then there as well is network_home_url(). And you got network_admin_url()

wp-content or the renamed directory

You can redefine the wp-content folder name. Therefore you use content_url() there.

How to get the plugins folder url?

If using WP_PLUGIN_DIR is wrong, what is the suggested alternative?

Simply use plugins_url() without any parameters.

If you use it for a plugin, it works for MU plugins too.

EDIT #1 If you’re interested in the path to the current plugin, use plugin_basename();.

EDIT #2 If you’re interested in all sidewide active plugins, use wp_get_active_network_plugins();.

If you’re not on multisite, go with wp_get_active_and_valid_plugins();. This will take multisite/network into account. Keep in mind that this will take them into account if you’re not on multisite, but got a sunrise.php dropin.

You could as well retrieve them via get_option( 'active_plugins' );, which is not recommended as it doesn’t take filters and plugin validation into account, which happens on several occasions.

Further reading

This list goes on and on. Take a look at the search results on QueryPosts.com for more info.

This article goes in absolute depth about all paths.