PHP RFC: Unary null coalescing operator


PHP 7.0 introduced the Null Coalescing Operator (??), which provides a convenient, more concise alternative to isset() or an explicit NULL check when retrieving the values of variables, properties and object members. Specifically, it provides the short syntax $foo ?? $bar which returns $foo if it exists and is not NULL, or otherwise $bar. This is useful when, for example, retrieving data from query parameters or a configuration file.

One use of the ?? operator is not to retrieve a value per se, but to check its value without worrying about whether it exists, for instance $_GET["action"] ?? NULL === "submit" lets you check if there is a query parameter named action that is set to submit, which is considerably shorter and less redundant than typing out isset($_GET["action"]) && $_GET["action"] === "submit".

It is this latter use-case that this RFC concerns. While ?? NULL is significantly better than the full expression using isset(), it is still redundant, since we have to provide some arbitrary default value. A shorter alternative would be to use the error-suppression operator (@), but it is slow and considered bad practice.

Thus, this RFC proposes a small tweak to ??.


This RFC proposes a unary version of ??, which would be equivalent to the normal binary version where the second argument is NULL. That is, $foo?? would now be valid, and behave identically to $foo ?? NULL in every respect.

The unary form of ?? would provide a faster, non-proscribed alternative to @ for retrieving possibly-unset variables. It would also provide a concise, non-redundant way to check the value of a possibly-unset variable.

One practical use is optional request parameters:

if ($_POST["action"]?? === "submit") {
    // Form submission logic
} else {
    // Form display logic

Another is optional options in, say, a configuration object, or an “options bag” parameter:

if ($optionsBag->safeMode?? === TRUE) {
    // Safe mode
} else {
    // Not safe

Backward Incompatible Changes


There is an ambiguity in the case where unary ?? is followed by an operator that can be either unary or binary, i.e. $a?? + $b and $a?? - $b. These continue to be parsed the same ($a ?? (+$b), $a ?? (-$b)), meaning there is no backwards-compatibility break.

Proposed PHP Version(s)

Next PHP 7.x, which would be PHP 7.2 at the time of writing.

RFC Impact

To SAPIs, Existing Extensions and Opcache

There is no effect on any of these, with the exception of extensions inspecting the AST, for which the unary ?? is indistinguishable from ?? NULL.

Unaffected PHP Functionality

The behaviour of binary ?? is unchanged, as is isset().

Future Scope



This is a simple language change which should only require a 2/3 majority vote on whether or not to approve it.

Voting started 2017-07-11 and ends 2017-07-18 ended 2017-07-18.

Accept and merge unary null coalescing operator for PHP 7.2?
Real name Yes No
aeoris (aeoris)  
ajf (ajf)  
ashnazg (ashnazg)  
bwoebi (bwoebi)  
dmitry (dmitry)  
emir (emir)  
galvao (galvao)  
jhdxr (jhdxr)  
kguest (kguest)  
krakjoe (krakjoe)  
levim (levim)  
mcmic (mcmic)  
nikic (nikic)  
ocramius (ocramius)  
peehaa (peehaa)  
zeev (zeev)  
Final result: 4 12
This poll has been closed.

Patches and Tests

A php-src patch, including a test, can be found here: https://github.com/php/php-src/pull/2589

A patch for the language specification, including the same test, can be found here: https://github.com/php/php-langspec/pull/197


After the project is implemented, this section should contain

  1. the version(s) it was merged to
  2. a link to the git commit(s)
  3. a link to the PHP manual entry for the feature
  4. a link to the language specification section (if any)


rfc/unary_null_coalescing_operator.txt · Last modified: 2017/07/19 02:31 by ajf