Problem with url in a pm

[Destroy666]

It's a function which works globally in MyBB (wherever inc/functions.php is included, so nearly everywhere, including inc/plugins files) and which is made to work with regular/SEF/Google SEO URLs. It should be used whenever possible for profile URLs (alternatively you can go with build_profile_url() if you want a regular HTML link), similarly get_post_link() for post links, get_thread_link() for thread links, etc.