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

$myVar = $someVar ?? 42;

Is equivalent to :

$myVar = isset($someVar) ? $someVar : 42;

For constants, the behaviour is the same when using a constant that already exists :

define("FOO", "bar");
define("BAR", null);

$MyVar = FOO ?? "42";
$MyVar2 = BAR ?? "42";

echo $MyVar . PHP_EOL;  // bar
echo $MyVar2 . PHP_EOL; // 42

However, for constants that don't exist, this is different :

$MyVar3 = IDONTEXIST ?? "42"; // Raises a warning
echo $MyVar3 . PHP_EOL;       // IDONTEXIST

Warning: Use of undefined constant IDONTEXIST - assumed 'IDONTEXIST' (this will throw an Error in a future version of PHP)

Php will convert the non-existing constant to a string.

You can use constant("ConstantName") that returns the value of the constant or null if the constant doesn't exist, but it will still raise a warning. You can prepended the function with the error control operator @ to ignore the warning message :

$myVar = @constant("IDONTEXIST") ?? "42"; // No warning displayed anymore
echo $myVar . PHP_EOL; // 42