PHP's null merge operator (??) is used to check whether a variable or array element exists and is not null. If it exists and has a value, it returns the value, otherwise it returns the specified default value. 1. It solves the problem of providing fallback values when undefined variables or null values, which is more concise and accurate than ternary operators (?:) and isset(); 2. Unlike ?:, ?? only triggers fallback when the value is null, and ?: will also trigger when the value is false (such as empty strings, 0, false); 3. It is often used to deal with default values for hyperglobal variables, optional array keys, class attributes, or function parameters; 4. Supports chain calls to try multiple options; 5. Note: Accessing undefined variables will still trigger notice, and you need to ensure that the parent variable exists.

The null coalescing operator ( ?? ) in PHP is a shortand way to check if a variable or array element exists and isn't null. If it exists and has a value, it returns that value; otherwise, it returns a default you specify.

What Problem Does ?? Solve?

Before ?? , developers often used the ternary operator ( ?: ) or isset() checks to provide fallback values. For example:

 $name = isset($_GET['name']) ? $_GET['name'] : 'Guest';

That works fine, but with ?? , the same line becomes cleaner:

 $name = $_GET['name'] ?? 'Guest';

It's especially useful when dealing with optional form inputs, configuration arrays, or user data where missing keys or null values are common.

This operator only triggers the fallback if the left-hand side is null — so even values like empty strings, 0, or false will be returned as-is.

How Is It Different From ?: ?

The ternary operator ( ?: ) checks for truthiness, not just nullness. That means if the value is something like 0 or an empty string ( '' ), it would also trigger the fallback:

 $value = '' ?: 'default'; // returns 'default'

With ?? , it only falls back if the value is actually null :

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

So if you're only concerned about nulls — like checking if an array key exists — ?? is more accurate and avoids unintended behavior.

When Should You Use ?? ?

Use it when working with:

• Superglobals like $_GET , $_POST , or $_SESSION
• Optional array keys in config files or API responses
• Fallback values in class properties or function parameters

Examples:

 $page = $_GET['page'] ?? 1;

$options = [
    'timeout' => 30,
    // 'retries' intentionally missing
];

$retryLimit = $options['retries'] ?? 5; // returns 5

You can also chain multiple ?? operators to try several options:

 $username = $_GET['user'] ?? $_POST['user'] ?? 'anonymous';

Just remember: this doesn't check whether a variable is defined. If you try to access an undefined variable entirely (not just set to null), PHP will throw a warning.

A Common Gotcha

Using ?? on an undefined variable still triggers a notice:

 echo $undefinedVar ?? 'fallback'; // This is OK
echo $undefinedArray['key'] ?? 'fallback'; // Also OK
echo $undefinedVar['key'] ?? 'fallback'; // Triggers a notice

In the last case, since $undefinedVar itself doesn't exist, trying to access an index of it causes PHP to complain — even though we're using ?? .

To safely handle nested values, make sure parent variables or arrays exist first.

Basically that's it.

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