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PHP Double Question Mark (??) Operator Explained

By Filip on 11/04/2024

Learn how to use the PHP double question mark (??) operator for concisely setting default values in your code.

PHP Double Question Mark (??) Operator Explained

Table of Contents

Introduction

The double question mark (??), known as the null coalescing operator in PHP, offers a streamlined approach to managing potentially undefined or null values. Introduced in PHP 7.0, it simplifies the process of providing default values when variables or expressions might not hold a defined value.

Step-by-Step Guide

The double question mark (??) in PHP is called the null coalescing operator. Introduced in PHP 7.0, it provides a concise way to handle potentially undefined or null values.

How it works:

  1. Evaluation: The operator checks if the value on its left side exists and is not null.
  2. Return:
    • If the left side is defined and not null, the operator returns that value.
    • If the left side is null or undefined, it returns the value on the right side.

Example:

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

In this example:

  • If $_GET['username'] is set (e.g., from a URL parameter) and not null, $username will be assigned that value.
  • If $_GET['username'] is not set or null, $username will be assigned the default value 'Guest'.

Benefits:

  • Conciseness: It replaces lengthier isset() checks and ternary operations.
  • Readability: Makes code cleaner and easier to understand.

PHP 7.4 and beyond:

PHP 7.4 introduced the null coalescing assignment operator (??=). This operator simplifies assigning a default value to a variable only if it's currently null.

Example:

$data['name'] ??= 'Unknown';

This assigns 'Unknown' to $data['name'] only if it's null. If $data['name'] already has a value, it remains unchanged.

Code Example

This code demonstrates the use of PHP's null coalescing operator (??) and null coalescing assignment operator (??=) for concisely handling potentially null values. It includes examples of providing default values for variables, safely accessing nested array elements, using the assignment operator for conditional assignment, and chaining the operator for multiple fallback values. The examples highlight how these operators improve code readability and efficiency compared to traditional methods like using isset() or ternary operators.

Here are some code examples demonstrating the null coalescing operator (??) and the null coalescing assignment operator (??=) in PHP:

1. Handling potentially undefined variables:

// Without null coalescing operator
$name = isset($_GET['name']) ? $_GET['name'] : 'Anonymous';

// With null coalescing operator
$name = $_GET['name'] ?? 'Anonymous';

echo "Hello, $name!";

2. Accessing nested array elements safely:

$user = [
    'profile' => [
        'city' => 'New York'
    ]
];

// Without null coalescing operator
$location = isset($user['profile']['country']) ? $user['profile']['country'] : 'Unknown';

// With null coalescing operator
$location = $user['profile']['country'] ?? 'Unknown';

echo "User is from: $location";

3. Using null coalescing assignment operator:

$data = [];

// Without null coalescing assignment operator
if (!isset($data['status'])) {
    $data['status'] = 'pending';
}

// With null coalescing assignment operator
$data['status'] ??= 'pending';

echo "Status: " . $data['status'];

4. Chaining null coalescing operators:

$value1 = null;
$value2 = 0;
$value3 = 'Hello';

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

echo $result; // Output: 0

Explanation:

  • In the first example, both code snippets achieve the same result: if $_GET['name'] is set, $name will be assigned that value; otherwise, it will be assigned 'Anonymous'. The null coalescing operator makes the code more concise.

  • The second example demonstrates accessing nested array elements safely. If $user['profile']['country'] doesn't exist, the null coalescing operator prevents an error and assigns 'Unknown' to $location.

  • The third example shows how the null coalescing assignment operator simplifies assigning a default value only if the variable is currently null.

  • The fourth example demonstrates chaining null coalescing operators. The code checks each value from left to right and returns the first one that is not null.

These examples showcase the benefits of using the null coalescing operator and its assignment counterpart in PHP for writing cleaner, more readable, and efficient code.

Additional Notes

  • The null coalescing operator (??) is a powerful tool for writing more concise and readable PHP code, especially when dealing with potentially undefined or null values.

  • It's important to note that the null coalescing operator only checks for null or undefined values. It won't handle empty strings, zero values, or boolean false.

  • The left-hand operand of the null coalescing operator is not evaluated twice, unlike using isset() in a ternary operation. This can be more efficient, especially when the left-hand operand is a function call or a complex expression.

  • While chaining null coalescing operators can be useful, it's essential to ensure the logic is clear and maintainable, especially as the number of chained operators increases.

  • The null coalescing assignment operator (??=) further streamlines the process of assigning default values to variables, making the code even more concise.

  • These operators are particularly helpful when working with arrays, superglobals like $_GET and $_POST, and database results that might return null values.

  • By adopting the null coalescing operator and its assignment counterpart, developers can write cleaner, more efficient, and easier-to-understand PHP code.

Summary

Feature Description
Operator ??
Introduced in PHP 7.0
Purpose Provides a concise way to handle potentially undefined or null values.
How it works - Checks if the left-hand operand exists and is not null.
- Returns the left-hand operand if it's defined and not null.
- Returns the right-hand operand if the left-hand operand is null or undefined.
Benefits - Conciseness: Replaces longer isset() checks and ternary operations.
- Readability: Makes code cleaner and easier to understand.
Example $username = $_GET['username'] ?? 'Guest';
(Assigns the value of $_GET['username'] to $username if it exists and is not null, otherwise assigns 'Guest'.)
PHP 7.4+ Enhancement Null Coalescing Assignment Operator (??=)
- Assigns the right-hand operand to the left-hand operand only if the left-hand operand is null.
PHP 7.4+ Example $data['name'] ??= 'Unknown';
(Assigns 'Unknown' to $data['name'] only if it's currently null.)

Conclusion

In conclusion, the null coalescing operator (??) and its companion, the null coalescing assignment operator (??=), introduced in PHP 7.0 and 7.4 respectively, provide valuable tools for modern PHP developers. They offer elegant and concise solutions for handling potentially undefined or null values, enhancing code readability and efficiency. By replacing verbose isset() checks and ternary operations with these operators, developers can write cleaner, more maintainable, and less error-prone code. These operators are particularly useful when working with user input, arrays, and database results, where null values are common. Embracing the null coalescing operator and its assignment counterpart empowers developers to write more robust and expressive PHP code.

References

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