What is null coalescing assignment ??= operator in PHP 7.4

In PHP 7 this was originally released, allowing a developer to simplify an isset() check combined with a ternary operator. For example, before PHP 7, we might have this code:

$data['username'] = (isset($data['username']) ? $data['username'] : 'guest');

When PHP 7 was released, we got the ability to instead write this as:

$data['username'] = $data['username'] ?? 'guest';

Now, however, when PHP 7.4 gets released, this can be simplified even further into:

$data['username'] ??= 'guest';

One case where this doesn't work is if you're looking to assign a value to a different variable, so you'd be unable to use this new option. As such, while this is welcomed there might be a few limited use cases.

From the docs:

Coalesce equal or ??=operator is an assignment operator. If the left parameter is null, assigns the value of the right paramater to the left one. If the value is not null, nothing is done.

Example:

// The folloving lines are doing the same$this->request->data['comments']['user_id'] = $this->request->data['comments']['user_id'] ?? 'value';// Instead of repeating variables with long names, the equal coalesce operator is used$this->request->data['comments']['user_id'] ??= 'value';

So it's basically just a shorthand to assign a value if it hasn't been assigned before.

The null coalescing assignment operator is a shorthand way of assigning the result of the null coalescing operator.

An example from the official release notes:

$array['key'] ??= computeDefault();// is roughly equivalent toif (!isset($array['key'])) {    $array['key'] = computeDefault();}