PHP Null Coalescing

What is Null Coalescing?

The null coalescing operator (??) provides a clean way to assign default values when a variable might be null or undefined. It returns the left operand if it exists and is not null, otherwise it returns the right operand.

$result = $variable ?? 'default';

// Equivalent to:
$result = isset($variable) ? $variable : 'default';

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Basic Usage

Simple Default Values

$username = null;
$user = $username ?? 'guest';
echo $user;  // "guest"

With Form Data

$name = $_POST['name'] ?? 'Anonymous';
$email = $_GET['email'] ?? '';
$age = $_POST['age'] ?? 0;

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Chaining

You can chain multiple ?? operators. PHP evaluates left to right and returns the first non-null value.

$theme = $_GET['theme'] ?? $_COOKIE['theme'] ?? $_SESSION['theme'] ?? 'light';

$dbHost = $_ENV['DB_HOST'] ?? $_SERVER['DB_HOST'] ?? 'localhost';

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With Arrays

Null coalescing is useful for accessing array keys that might not exist:

$config = ['database' => 'mysql', 'port' => 3306];

$host = $config['host'] ?? 'localhost';      // 'localhost'
$timeout = $config['timeout'] ?? 30;         // 30
$ssl = $config['ssl'] ?? false;              // false

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Assignment Operator (??=)

The ??= operator (PHP 7.4+) assigns a value only if the variable is null or undefined:

$settings = [];

$settings['language'] ??= 'en';
$settings['timezone'] ??= 'UTC';

// If already set, value won't change
$settings['language'] = 'fr';
$settings['language'] ??= 'en';
echo $settings['language'];  // Still 'fr'

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Practical Examples

Configuration with Fallbacks

class Config {
    private $settings = [];

    public function get($key, $default = null) {
        return $this->settings[$key] ?? $default;
    }
}

$config = new Config();
$apiUrl = $config->get('api_url', 'https://api.example.com');

Handling API Responses

$response = [
    'user' => [
        'name' => 'Alice',
        'email' => 'alice@example.com'
    ]
];

$name = $response['user']['name'] ?? 'Unknown';
$phone = $response['user']['phone'] ?? 'Not provided';

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Comparison with Other Operators

The key difference: ?? only checks for null/undefined, while || checks for all falsy values.

$empty = '';
$zero = 0;
$null = null;

// Null coalescing (??) - only null/undefined triggers default
echo $empty ?? 'default';  // '' (empty string is not null)
echo $zero ?? 'default';   // 0 (zero is not null)
echo $null ?? 'default';   // 'default'

// Logical OR (||) - any falsy value triggers default
echo $empty || 'default';  // 'default' (empty string is falsy)
echo $zero || 'default';   // 'default' (zero is falsy)
echo $null || 'default';   // 'default'

Use ?? when you want to allow empty strings, zero, or false as valid values. Use || when you want to replace any falsy value.