Closures as callbacks in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a closure used as a callback

What is the output of this PHP code using a closure as a callback?

PHP

<?php
$numbers = [1, 2, 3, 4];
$result = array_map(function($n) {
    return $n * 2;
}, $numbers);
print_r($result);
?>

A Array ( [0] => 2 [1] => 4 [2] => 6 [3] => 8 )

B Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 4 )

C Array ( [0] => 1 [1] => 4 [2] => 9 [3] => 16 )

D Fatal error: Uncaught Error: Call to undefined function array_map()

💡 Hint

array_map applies the callback to each element and returns a new array.

The closure doubles each number in the array. So 1 becomes 2, 2 becomes 4, and so on.

❓ Predict Output

intermediate

Closure capturing external variable

What will be the output of this PHP code where a closure captures an external variable?

PHP

<?php
$factor = 3;
$numbers = [1, 2, 3];
$result = array_map(function($n) use ($factor) {
    return $n * $factor;
}, $numbers);
print_r($result);
?>

A Array ( [0] => 0 [1] => 0 [2] => 0 )

B Array ( [0] => 3 [1] => 6 [2] => 9 )

C Fatal error: Uncaught Error: Undefined variable: factor

D Array ( [0] => 1 [1] => 2 [2] => 3 )

💡 Hint

The closure uses the 'use' keyword to access $factor inside.

The closure multiplies each number by 3, the value of $factor captured from outside.

❓ Predict Output

advanced

Modifying external variable inside closure

What is the output of this PHP code where a closure modifies an external variable by reference?

PHP

<?php
$sum = 0;
$numbers = [1, 2, 3];
array_walk($numbers, function($n) use (&$sum) {
    $sum += $n;
});
echo $sum;
?>

A 6

B 0

C Fatal error: Uncaught Error: Cannot use variable by reference

D 3

💡 Hint

The closure uses & to modify $sum by reference.

The closure adds each number to $sum, so the total is 1+2+3=6.

❓ Predict Output

advanced

Closure as callback with default argument

What will this PHP code output when using a closure with a default argument as a callback?

PHP

<?php
$numbers = [1, 2, 3];
$result = array_map(function($n, $multiplier = 2) {
    return $n * $multiplier;
}, $numbers);
print_r($result);
?>

A Warning: array_map() expects at most 2 parameters, 3 given

B Array ( [0] => 2 [1] => 4 )

C Array ( [0] => 2 [1] => 4 [2] => 6 )

D Array ( [0] => 1 [1] => 2 [2] => 3 )

💡 Hint

array_map passes only one argument to the callback here.

array_map passes only one argument ($n) to the callback, so $multiplier defaults to 2. Each number is doubled: 1→2, 2→4, 3→6.

🧠 Conceptual

expert

Why use closures as callbacks in PHP?

Which is the best explanation for why closures are useful as callbacks in PHP?

A Closures automatically optimize code performance by compiling callbacks to machine code.

B Closures are required in PHP because named functions cannot be used as callbacks.

C Closures prevent any variable from being accessed inside the callback, ensuring strict isolation.

D Closures allow you to write inline functions that can capture variables from the surrounding scope, making callbacks more flexible and concise.

💡 Hint

Think about how closures can access variables outside their own function body.

Closures let you create small functions on the fly that can use variables from where they were created, which is very handy for callbacks.