Closures in array functions in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array_map with closure capturing variable

What is the output of the following PHP code?

PHP

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

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

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

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

D Fatal error: Undefined variable: multiplier

💡 Hint

Closures can capture variables from the surrounding scope using the 'use' keyword.

The closure captures the variable $multiplier by value using 'use ($multiplier)'. Each element in the array is multiplied by 3, producing [3, 6, 9].

❓ Predict Output

intermediate

Effect of modifying captured variable by reference in closure

What will be the output of this PHP code?

PHP

<?php
$factor = 2;
$numbers = [1, 2, 3];
$result = array_map(function($n) use (&$factor) {
    return $n * $factor;
}, $numbers);
$factor = 5;
print_r($result);
?>

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

B Array ( [0] => 5 [1] => 10 [2] => 15 )

C Array ( [0] => 10 [1] => 20 [2] => 30 )

D Fatal error: Cannot use variable by reference in closure

💡 Hint

The closure captures the variable by reference, but array_map applies the function immediately.

The closure uses $factor by reference, but array_map calls the closure immediately with $factor = 2. Changing $factor after array_map does not affect the already computed result.

🔧 Debug

advanced

Identify the error in closure used with array_filter

What error does the following PHP code produce?

PHP

<?php
$threshold = 10;
$values = [5, 15, 20];
$filtered = array_filter($values, function($v) {
    return $v > $threshold;
});
print_r($filtered);
?>

A Array ( [0] => 5 [1] => 15 [2] => 20 )

B Parse error: syntax error, unexpected '{'

C Array ( [1] => 15 [2] => 20 )

D Fatal error: Undefined variable: threshold

💡 Hint

Variables outside the closure must be imported with 'use' to be accessible inside.

The closure does not have access to $threshold because it is not imported with 'use'. This causes an undefined variable error.

📝 Syntax

advanced

Syntax error in closure with array_reduce

Which option correctly fixes the syntax error in this PHP code?

PHP

<?php
$numbers = [1, 2, 3];
$sum = array_reduce($numbers, function($carry, $item) {
    $carry += $item
    return $carry;
});
echo $sum;
?>

A Remove the return statement inside the closure

B Add a comma after '$carry += $item,' inside the closure

C Add a semicolon after '$carry += $item;' inside the closure

D Change 'function' to 'fn' for arrow function syntax

💡 Hint

PHP statements must end with semicolons.

The missing semicolon after '$carry += $item' causes a syntax error. Adding it fixes the code.

🚀 Application

expert

Count how many elements satisfy a condition using closure

Given the array $data = [2, 7, 4, 9, 3], which option correctly counts how many numbers are greater than 5 using array_filter and a closure?

PHP

<?php
$data = [2, 7, 4, 9, 3];
$count = /* fill in here */;
echo $count;
?>

A array_reduce($data, fn($c, $x) => $c + ($x > 5), 0)

B count(array_filter($data, fn($x) => $x > 5))

C array_filter($data, fn($x) => $x > 5)

D count(array_map(fn($x) => $x > 5, $data))

💡 Hint

array_filter returns filtered elements; count counts them.

Option B filters elements greater than 5 and counts them, producing the correct count 2. Option B returns an array, not a count. Option B counts booleans (all 5). Option B sums booleans but is not asked for.