After reading through the PHP manual, I added session_name('mysite'); to my code to ensure that collisions between sessions won’t happen if I will run multiple apps in the future.
Unfortunately the session_name()-function call it kills my site completely . A fatal error is thrown (which appears in the error logs, but doesn’t say anything!) and the below error is shown in the browser:
I have PHP-FPM running along with suhosin+nginx at a freshly installed Ubuntu VM. What am I doing wrong?

As pointed out in the comments on the session_name doc page, there are some issues w/ session_name that can cause your site to break:
session_namebefore anything else, includingsession_start