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?
EDIT:
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:
http://php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op
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.
Answer
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.