PHP Null Coalescing Operators

The operators ??, ?: and ??= in PHP are all null coalescing operators and have similar functions.

Take the following two variables.

$one = NULL;
$two = 'two';

We could set a default value using an if statement.

if (isset($one) && $one !== NULL) {
  $result = $one;
}
else {
  $result = $two;
}

The variable $result now contains the value 'two' since the variable $one is null.

This is equivalent to the following, using the ?? operator.

$result = $one ?? $two;

We can set the value of $one with the value of $two if $one is null by using the ??= operator.

$one ??= $two;

The ternary operator can also be used to set the value of $result, depending on the value of $one and $two.

$result = $one ? $one : $two;

This can be shortened using the ?: (or Elvis) operator.

$result = $one ?: $two;

The Elvis operator is useful when looking at the return of some internal PHP functions, since they can either produce a result, or false in the correct circumstances.

For example, the strstr() function in PHP is used to find the position of a string in another string. The issue, however, is that the function returns false if that string isn't found. This sort of makes sense since you wouldn't return 0 as that would mean the string was found at position 0.

We can, however, correct the output here by setting a binary constant value and returning that if the string wasn't found in the comparison.

const STRING_NOT_FOUND = 0b00111;
$result = strstr('one', 'two') ?: STRING_NOT_FOUND;

The value of $result is now the constant STRING_NOT_FOUND.

These operators can be chained to give multiple default values.

For example, we can set the value of $result based on what values exist in the various ways in which the user can submit data to the system using the ?? operator.

$result = $_GET['value'] ?? $_POST['value'] ?? $_SESSION['value'] ?? $_COOKIE['user'] ?? '';

Here, the call to $_GET['value'] and others will not cause any warnings to be triggered due to these values not being set.

In the case of the ?: operator, although the same result is seen, the evaluation of the different arrays causes PHP warning to be produced.

$result = $_GET['value'] ?: $_POST['value'] ?: $_SESSION['value'] ?: $_COOKIE['user'] ?: '';

If the variables exist, like in our original two values, then no errors are produced.

$result = $one ?: $two ?: '';

Whilst is it not advisable to "stack" ternary operators due to the result being difficult to read. Null coalescing operators are safe to stack in this way.