What does double question mark (??) operator mean in PHP

I was diving into Symfony framework (version 4) code and found this piece of code: $env = $_SERVER['APP_ENV'] ?? 'dev'; I'm not sure what this actually does but I imagine that it expands to something like: ```php $env = $_SERVER['APP_ENV'] != null ? $_SERVER['APP_ENV'] : 'dev'; ``` Or maybe: ```php $env = isset($_SERVER['APP_ENV']) ? $_SERVER['APP_ENV'] : 'dev'; ``` Does someone have any precision about the subject? Note: the main difference between my question and [this question][1] is that most of people (like me before) ignore that the double question mark is called null coalescing operator. Thus, this question is more relevant and accessible to beginners. [1]: https://stackoverflow.com/questions/34571330/php-short-ternary-elvis-operator-vs-null-coalescing-operator