![What Does Double Question Mark (? ?) Operator Mean in Php [Duplicate]](https://docuprint.cloud68.co/uploads/what-does-double-question-mark-operator-mean_1200.jpg)
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:
$env = $_SERVER['APP_ENV'] != null ? $_SERVER['APP_ENV'] : 'dev';
Or maybe:
$env = isset($_SERVER['APP_ENV']) ? $_SERVER['APP_ENV'] : 'dev';
Does someone have any precision about the subject?
3 Answers
It's the "null coalescing operator", added in php 7.0. The definition of how it works is:
It returns its first operand if it exists and is not NULL; otherwise it returns its second operand.
So it's actually just isset() in a handy operator.
Those two are equivalent1:
$foo = $bar ?? 'something';
$foo = isset($bar) ? $bar : 'something';
Documentation:
In the list of new PHP7 features:
And original RFC
EDIT: As this answer gets a lot of views, little clarification:
1There is a difference: In case of ??, the first expression is evaluated only once, as opposed to ? :, where the expression is first evaluated in the condition section, then the second time in the "answer" section.
$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
$x = $y ?? 'dev'
is short hand for x = y if y is set, otherwise x = 'dev'
There is also
$x = $y =="SOMETHING" ? 10 : 20
meaning if y equals 'SOMETHING' then x = 10, otherwise x = 20
