Null Coalescing Operator

Part of the Fundamentals section of Coddy's PHP journey — lesson 31 of 71.

The null coalescing operator (??) provides a convenient way to use a default value when a variable is null or doesn't exist. This is especially useful when working with data that might be missing.

The syntax places ?? between the variable to check and the fallback value:

<?php
$username = null;

$displayName = $username ?? "Guest";
echo $displayName; // Guest
?>

PHP checks if $username is null. Since it is, the operator returns "Guest" instead. If $username had a value, that value would be used.

<?php
$username = "Alice";

$displayName = $username ?? "Guest";
echo $displayName; // Alice
?>

The null coalescing operator also works when a variable hasn't been defined at all, without triggering an error:

<?php
// $color is not defined anywhere
$selectedColor = $color ?? "blue";
echo $selectedColor; // blue
?>

This makes ?? cleaner than using a ternary operator with isset() checks. It's perfect for providing sensible defaults when data might be missing.

Challenge

Easy

Read two values from input: a string nickname and a string theme.

Use the null coalescing operator to handle potentially missing values:

Create a variable $userNickname that is set to null. Then use the null coalescing operator to assign $displayNickname the value of $userNickname if it exists and is not null, otherwise use the nickname input value.
Create a variable $userTheme and assign it the theme input value. Use the null coalescing operator to assign $selectedTheme the value of $userTheme if it exists and is not null, otherwise use dark as the default.
Use the null coalescing operator directly with an undefined variable $language (do not define it) to assign $displayLanguage the value of $language if it exists, otherwise use English as the default.

Print each result on a new line in this order: $displayNickname, $selectedTheme, $displayLanguage.

Example:

If the inputs are Player1 and light, the output should be:

Player1
light
English

If the inputs are GuestUser and blue, the output should be:

GuestUser
blue
English

Cheat sheet

The null coalescing operator (??) provides a default value when a variable is null or doesn't exist.

Syntax:

$result = $variable ?? "default value";

If $variable is null, the default value is used:

$username = null;
$displayName = $username ?? "Guest";
echo $displayName; // Guest

If $variable has a value, that value is used:

$username = "Alice";
$displayName = $username ?? "Guest";
echo $displayName; // Alice

The operator also works with undefined variables without triggering an error:

// $color is not defined
$selectedColor = $color ?? "blue";
echo $selectedColor; // blue

Try it yourself

<?php
// Read input
$nickname = trim(fgets(STDIN));
$theme = trim(fgets(STDIN));

// TODO: Write your code below
// 1. Create $userNickname set to null, then use ?? to assign $displayNickname
// 2. Create $userTheme with the theme input, then use ?? to assign $selectedTheme (default: 'dark')
// 3. Use ?? with undefined $language to assign $displayLanguage (default: 'English')

// Output the results (each on a new line)
echo $displayNickname . "\n";
echo $selectedTheme . "\n";
echo $displayLanguage . "\n";
?>

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Fundamentals

2Variables and Data Types

Numbers Strings and Quotes Booleans Naming Conventions Recap - Variable Init Empty Variables String Concatenation Getting User Input Cast to Different Types

5Conditional Logic

If Statement If - Else The Ternary OperatorNull Coalescing OperatorSwitch Statement Recap - Making Decisions

8Loops

For Loop While Loop Foreach Loop Looping with Keys and Values Break Statement Continue Statement Recap - Iterating Over Data

11Final Challenges

Challenge - Factorial Calc Challenge - Longest Word Challenge - Vowel Counter