What Does The ?? Operator Mean In PHP?

Definition and Functionality

The ?? operator, or null coalescing operator, was added in PHP 7.0. It returns the first non-null value in a series of expressions. This operator helps write shorter code when working with undefined or null values.

Syntax and Usage

The basic syntax of the ?? operator is:

$result = $variable ?? 'default_value';

This expression checks from left to right and stops at the first non-null value. If $variable is set and not null, it's assigned to $result. If not, 'default_value' is assigned.

You can chain multiple expressions with the ?? operator:

$result = $value1 ?? $value2 ?? $value3 ?? 'default_value';

Here, the operator checks each value in order until it finds a non-null value or reaches the final default value.

The ?? operator is useful when working with arrays, user input, or any situation where a variable might not be set. It lets you provide a fallback value without using longer conditional statements.

$username = $_POST['username'] ?? 'Guest';
echo "Welcome, $username!";

Simplifying Variable Assignments

The ?? operator helps set default values in PHP. It reduces isset() checks, making your code cleaner. For example:

$config = [
    'debug' => true,
    'cache_time' => 3600
];

$debug_mode = $config['debug'] ?? false;
$cache_duration = $config['cache_time'] ?? 1800;

If 'debug' is not set in the $config array, $debug_mode will be false. If 'cache_time' is not set, $cache_duration will be 1800.

$preferred_language = $_GET['lang'] ?? $_SESSION['lang'] ?? 'en';

Working with Arrays and Object Properties

The ?? operator helps when working with arrays and object properties. It lets you safely access undefined array keys or nullable object properties.

For arrays:

$user = [
    'name' => 'John',
    'age' => 30
];

$user_name = $user['name'] ?? 'Unknown';
$user_email = $user['email'] ?? 'No email provided';

$user_name will be 'John', but $user_email will be 'No email provided' since the 'email' key doesn't exist in the $user array.

For object properties:

class User {
    public $name;
    public $email;
}

$user = new User();
$user->name = 'Jane';

$name = $user->name ?? 'Unknown';
$email = $user->email ?? 'No email set';

$name will be 'Jane', but $email will be 'No email set' because the email property was not set on the $user object.

The ?? operator simplifies these tasks in PHP, making your code shorter and easier to read.

?? vs. Ternary Operator (?:)

The ?? (null coalescing) operator and the ?: (ternary) operator are used for conditional assignments in PHP, but they have differences.

The ternary operator checks if a condition is true, while the null coalescing operator checks if a value is null or undefined. Here's a comparison:

// Ternary operator
$result = (condition) ? value_if_true : value_if_false;

// Null coalescing operator
$result = $value ?? default_value;

The null coalescing operator is shorter when you want to check if a value is set and not null. It's useful for array key checks and default value assignments.

Use the ternary operator when you need to evaluate a specific condition beyond checking for null or undefined values. Use the null coalescing operator when you want to provide a default value for potentially null or undefined variables.

?? vs. Null Coalescing Assignment Operator (??=)

PHP 7.4 introduced the ??= (null coalescing assignment) operator. This operator combines assignment with the null coalescing check.

The ??= operator assigns the right operand to the left operand only if the left operand is null or undefined. Here's how it works:

// Without ??=
$value = $value ?? 'default';

// With ??=
$value ??= 'default';

Both examples do the same thing: if $value is null or undefined, it's assigned the value 'default'.

The ??= operator is useful when you want to set a default value for a variable only if it hasn't been set before. It's helpful in scenarios like initializing configuration values or setting default properties in objects.

$config = [];
$config['debug'] ??= false;
$config['environment'] ??= 'production';

In this example, 'debug' and 'environment' are only set if they don't already exist in the $config array.

The ??= operator simplifies code in situations where you previously might have used isset() checks or longer conditional statements to initialize variables with default values.

class User {
    private $name;
    private $role;

    public function __construct($name = null, $role = null) {
        $this->name ??= 'Guest';
        $this->role ??= 'Visitor';
    }
}