What is the null coalescing operator and how does it work?

The null coalescing operator ?? returns its left operand if it exists and is not null, and otherwise returns the right operand. PHP 7.0 added it as a short form of a ternary with isset(). It is handy for default values.

$username = $_GET['user'] ?? 'guest';
// Equivalent to:
$username = isset($_GET['user']) ? $_GET['user'] : 'guest';

Like isset(), it raises no warning when the key or variable is missing. It can be chained, so $_GET['user'] ?? $_POST['user'] ?? 'guest' returns the first value that is set.

PHP 7.4 added the null coalescing assignment operator ??=. It assigns only when the left side is missing or null, as in $config['timeout'] ??= 30.