How to include a plugin’s php file to another plugin functions file [duplicate]

I need to customize the output of the shortcode of plugin A by using a function of plugin B. So I alter the shortcode function of plugin A by inserting a condition to check the value of the function of plugin B but I need to include the php file that supports this function. I tried all require_once, require and include but I get the following errors:

When using
require_once('../../pluginname/pluginfunctions.php');

Error
Warning: require_once(): open_basedir restriction in effect. File(../../magicmembers/core/libs/functions/mgm_misc_functions.php) is not within the allowed path(s)

When using
include(WP_PLUGIN_URL . '/pluginname/pluginfunctions.php');

Error
Warning: include(): URL file-access is disabled in the server configuration

What is the correct way?

The first error message means that there is restrictions in place on where you can include files from, set by the server. You could try with

require_once ABSPATH . '/wp-content/plugins/pluginname/pluginfunctions.php';

but I’m not sure if it would work.

With the second include you’re trying to include an URL which is disabled by the server for security reasons.

However, why do you need to include the function of plugin B? If plugin B is present that means it’s probably activated, which in turn means you can use the function directly from plugin A without needing to include the file specifically.