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

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?


To all the people who marked my question as negative because there's already a similar question (PHP ternary operator vs null coalescing operator):

It is true that both questions are very similar. However it is hard for everybody to imagine that the "??" is called the coalescing operator.

Otherwise I could easy find it on the official documentation:


However, for someone who didn't know that this feature was added in php 7 it's more likely to type:

"php ?? operator" or "php double question mark operator"

And here is why my question has an added value.

This question is related to php operator-keyword

The answer is

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.

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