2017
Use mu-plugins for adding custom functionality to your WordPress site
If you google “functions.php” you get about 7 million results. I bet most of them contain bad advice: “How to add functionality to your WordPress site”. Some of them continue even worse: “\[…\] without using a plugin”. For your own good, don’t edit functions.php to add custom functionality to your WordPress site. You can use mu-plugins to do that.