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

Question

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?

Solution 1

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: http://php.net/manual/en/language.operators.comparison.php#language.operators.comparison.coalesce

In the list of new PHP7 features: http://php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op

And original RFC https://wiki.php.net/rfc/isset_ternary

---

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.

Solution 2

$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

Solution 3

$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

Comments

[ascsoftw]

$foo = isset($y++) ? $y++ : 'something'; This doesn't work. Can not use isset on the result of an expression.

[michalhosna]

@ascsoftw sorry, removed until i find of better example

[Tomek]

Regarding the last edit : the number of times it gets evaluated also depends on the internal implementation of the ?? operator (is it really once?)

[ksadowski]

+1 for the documentation link. I end up here every now and then because the operator has to be spelled out for Google to treat it as a search term, but all I really want is to read the docs ;)

[Valentin]

Last edit is not very clear/concise. I had hard time reading and understanding it. Found this which clears what you wanted to say there. So please add an example. php.net/manual/en/language.operators.logical.php#115208 $a = (fruit(1) ? fruit(1) : 'apple');//fruit() will be called twice!

[Michiel Bakker]

Note that, like with isset(), this also works on undefined (e.g. typed class properties that have not been inialized yet).

[Semra]

The edit is really unnecessary and confusing. The syntax for a??b, a?:b, and a?a:b all clear enough that only the last one has duplicate evaluation

[AaA]

so does $value = $value ?? null; means something? (A line from google cloud api source code)

[michalhosna]

@AaA I covers the case where the variable $value did not exist at all.

[Kamil Dąbrowski]

if(($_SESSION['captchaReq']++??$_SESSION['captchaReq']=0)<3) { }

[Cid]

@KamilDąbrowski yes? What's this?

[Cid]

If you can't, don't bother with shortening code. A code you can read and maintain easily is way better than something short

[Kamil Dąbrowski]

$_SESSION['req']++??$_SESSION['req']=0 is easy to read take advantage of new technologies

[Cid]

well, you can't do null++, so doing $_SESSION['req']++?? makes no sense

[Kamil Dąbrowski]

you don't understand code is not null is 0 as begin initial any variable read again this ---> $_SESSION['req']++??$_SESSION['req']=0 You can image a lot think todo with the construction of logick avaiable from php 7> $hitCache++??$hitCache=0; it is post

[Cid]

What the point of using a null coalescing operator on something that can't be null?