What Is the Null Coalescing Operator in PHP?

Introduced in PHP 7, the null coalescing operator (??) has the following syntax:

// PHP 7+
leftExpr ?? rightExpr;

Which means that leftExpr is returned when leftExpr exists and is NOT null; otherwise it returns rightExpr. For example, all the following statements are equivalent:

// PHP 7+
// using the null coalescing operator
$x ?? $y;

// using the ternary operator
isset($x) ? $x : $y;

// using if/else
if (isset($x)) {
    return $x;
} else {
    return $y;
}

Please note that the null coalescing operator only evaluates to the right-hand side operand if the left-hand side operand is null or not set. This means that all other falsy values return the left-hand side operand.

The coalescing can also be chained (i.e. it returns the first defined/non-null value it encounters), for example:

$x ?? $y ?? $z ?? 'empty'; // output: 'empty'

In case there's no defined value in the coalescing chain, a "Notice: Undefined variable: ..." message is shown.

Starting with PHP 7.4, you can also use the null coalescing assignment operator ((??=)).