Null coalescing operator in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of null coalescing with undefined variable

What is the output of this PHP code snippet?

PHP

<?php
$a = null;
echo $a ?? 'default';
?>

A default

B null

C Notice: Undefined variable

D 0

💡 Hint

The null coalescing operator returns the right side if the left is null or undefined.

Since $a is explicitly set to null, the null coalescing operator returns the right side 'default'.

❓ Predict Output

intermediate

Null coalescing with multiple variables

What will this PHP code output?

PHP

<?php
$x = null;
$y = 0;
$z = 'hello';
echo $x ?? $y ?? $z;
?>

A hello

B 0

C None

D Notice: Undefined variable

💡 Hint

The operator returns the first value that is not null.

Here, $x is null, so it checks $y which is 0 (not null), so it outputs 0.

❓ Predict Output

advanced

Null coalescing with array keys

What is the output of this PHP code?

PHP

<?php
$arr = ['a' => null, 'b' => 5];
echo $arr['a'] ?? $arr['b'] ?? 10;
?>

A 10

B null

C 5

D Notice: Undefined index

💡 Hint

Null coalescing operator checks if the key exists and is not null.

The key 'a' exists but is null, so it moves to 'b' which is 5 and outputs 5.

❓ Predict Output

advanced

Null coalescing with undefined array key

What will this PHP code output?

PHP

<?php
$arr = ['x' => 1];
echo $arr['y'] ?? 'missing';
?>

A missing

B Notice: Undefined index

C 1

D null

💡 Hint

Null coalescing operator avoids notice for undefined keys.

Since 'y' key does not exist, it returns the right side 'missing' without error.

❓ Predict Output

expert

Null coalescing with function return values

What is the output of this PHP code?

PHP

<?php
function getValue() {
    return false;
}

$result = getValue() ?? 'default';
echo $result;
?>

A Notice: Undefined variable

B default

C 0

D false

💡 Hint

Null coalescing checks for null, not false or zero.

The function returns false which is not null, so the operator returns false and echoes it as empty string.