Using WP_PLUGIN_DIR for include file

Question

I hvae a plugin which includes php file from another plugin.

So I would do this:

plugin 'foo' includes file from plugin 'blah':

wp-content/plugins/foo

include(__DIR__."/../blah/some-file.php");  

This works but I am sure if I like this ../ , I wanted to reference plugin directory like:

include( WP_PLUGIN_DIR.'/blah/some-file.php' );

This works as well, but it say here:

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.

https://codex.wordpress.org/Determining_Plugin_and_Content_Directories

I cannot use plugin_dir_url() without an argument, like:

include( plugin_dir_url().'/blah/some-file.php' );

I can only do this:

include( plugin_dir_url(__FILE__).'/../blah/some-file.php' );

which is the same as first include above with DIR

Answer 1

It doesn't make a lot of sense, to me, to say "these should not be used directly by plugins". I tend to think you should use an available function over the constant - if an available function exists - which, in your case, does not. I would go ahead and use the WP_PLUGIN_DIR constant.