get server path of wp-includes

Hi, I’m trying to add a reCAPTCHA to a form I’ve created. I’ve looked at various WordPress plugins, and they all lack in one aspect or another. The choice to use the PHP library has been made, and it doesn’t seem too complicated.

I’ve decided that putting the library in wp-includes would be the most proper place, but I’m not sure if it’s okay to be using a server path like $_SERVER[‘DOCUMENT_ROOT’] – will this always give me the root of the wp install, even if its moved around to subdirectories? Is there a built-in wp function to get the wp-includes folder?

Thanks in advance!