Q:

What is the 'Null coalesce' (??) operator used for?

With the release of a new PHP version, PHP 7, new features are introduced. among these new features is an operator I am not familiar with. The Null coalesce operator.
What is this operator and what are some good use cases?

A:

You can use it to init a variable that might be null

The ?? operator is called the null-coalescing operator. It returns the
  left-hand operand if the operand is not null; otherwise it returns the
  right hand operand.

Source: https://msdn.microsoft.com/nl-nl/library/ms173224.aspx
(not dependent on language)
Use case
You can write
$rabbits;

$rabbits = count($somearray);

if ($rabbits == null) {
    $rabbits = 0;
}

You can use the shorter notation
$rabbits = $rabbits ?? 0;

