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.

Practice

1. What is the main purpose of using a closure inside PHP array functions like array_map or array_filter?

easy

A. To define a small, unnamed function that processes array elements inline

B. To declare a global variable accessible inside the array

C. To create a new array without any function

D. To convert arrays into strings automatically

Solution

1. Step 1: Understand what a closure is

   A closure is a small, unnamed function that can be used directly where needed.

2. Step 2: Role in array functions

   Closures let you write the logic for processing each array element inline, making code concise and clear.

3. Final Answer:

   To define a small, unnamed function that processes array elements inline -> Option A

4. Quick Check:

   Closure = small inline function [OK]

Hint: Closures are small functions used directly inside array calls [OK]

Common Mistakes:

• Thinking closures declare global variables
• Confusing closures with array conversion functions
• Assuming closures create arrays without functions

2. Which of the following is the correct syntax to use a closure with array_filter to keep only even numbers from an array $numbers?

easy

A. array_filter($numbers, function $n { return $n % 2 == 0; });

B. array_filter($numbers, function($n) { return $n % 2 == 0; });

C. array_filter($numbers, function($n) => $n % 2 == 0;);

D. array_filter($numbers, function($n) { $n % 2 == 0; });

Solution

1. Step 1: Check closure syntax

   Correct closure syntax uses function($param) { return expression; }.

2. Step 2: Validate each option

   array_filter($numbers, function($n) { return $n % 2 == 0; }); uses correct syntax with return statement. array_filter($numbers, function $n { return $n % 2 == 0; }); misses parentheses. array_filter($numbers, function($n) => $n % 2 == 0;); uses arrow syntax incorrectly. array_filter($numbers, function($n) { $n % 2 == 0; }); lacks return keyword.

3. Final Answer:

   array_filter($numbers, function($n) { return $n % 2 == 0; }); -> Option B

4. Quick Check:

   Closure syntax needs parentheses and return [OK]

Hint: Closures need parentheses and return keyword inside curly braces [OK]

Common Mistakes:

• Omitting parentheses around parameters
• Using arrow syntax incorrectly in PHP 7+
• Forgetting to return a value inside closure

3. What will be the output of the following PHP code?

$nums = [1, 2, 3, 4];
$result = array_map(function($x) { return $x * 3; }, $nums);
print_r($result);

medium

A. [0, 3, 6, 9]

B. [1, 2, 3, 4]

C. [3, 6, 9, 12]

D. Syntax error

Solution

1. Step 1: Understand array_map with closure

   array_map applies the closure to each element, multiplying by 3.

2. Step 2: Calculate each element

   1*3=3, 2*3=6, 3*3=9, 4*3=12, so result is [3, 6, 9, 12].

3. Final Answer:

   [3, 6, 9, 12] -> Option C

4. Quick Check:

   Multiply each by 3 = [3,6,9,12] [OK]

Hint: array_map applies closure to each element and returns new array [OK]

Common Mistakes:

• Expecting original array unchanged
• Confusing array_map with array_filter
• Thinking output is a string, not array

4. Identify the error in this PHP code using a closure with array_filter:

$values = [10, 15, 20];
$filtered = array_filter($values, function($v) {
  if ($v > 10) {
    return true;
  }
});
print_r($filtered);

medium

A. The closure does not return false for values <= 10, causing unexpected filtering

B. The array_filter function requires a named function, not a closure

C. The array_filter call is missing the array parameter

D. The closure syntax is invalid without a semicolon after return

Solution

1. Step 1: Analyze closure return behavior

   The closure returns true only if value > 10, but returns nothing (null) otherwise.

2. Step 2: Understand array_filter behavior

   array_filter expects explicit boolean returns. Returning implicit null (falsy) for values <=10 removes them but can cause unexpected filtering behavior.

3. Final Answer:

   The closure does not return false for values <= 10, causing unexpected filtering -> Option A

4. Quick Check:

   Closure must return true or false explicitly [OK]

Hint: Always return true or false explicitly in closures for array_filter [OK]

Common Mistakes:

• Not returning false explicitly in closure
• Thinking closures must be named functions
• Missing array parameter in array_filter

5. Given an array $data = [1, 2, 3, 4, 5, 6], how can you use a closure with array_filter and array_map together to get an array of squares of only the odd numbers?

hard

A. array_filter(fn($x) => $x % 2 !== 0, array_map($data, fn($x) => $x * $x));

B. array_filter(array_map($data, fn($x) => $x * $x), fn($x) => $x % 2 !== 0);

C. array_map(array_filter($data, fn($x) => $x % 2 !== 0), fn($x) => $x * $x);

D. array_map(fn($x) => $x * $x, array_filter($data, fn($x) => $x % 2 !== 0));

Solution

1. Step 1: Filter odd numbers first

   Use array_filter with closure to keep only odd numbers: fn($x) => $x % 2 !== 0.

2. Step 2: Map squares of filtered numbers

   Apply array_map with closure to square each filtered number: fn($x) => $x * $x.

3. Step 3: Combine calls correctly

   Pass filtered array as second argument to array_map, so filtering happens before mapping.

4. Final Answer:

   array_map(fn($x) => $x * $x, array_filter($data, fn($x) => $x % 2 !== 0)); -> Option D

5. Quick Check:

   Filter odds then map squares [OK]

Hint: Filter first, then map to transform filtered results [OK]

Common Mistakes:

• Reversing order of array_map and array_filter
• Passing arguments in wrong order
• Using array_map or array_filter with wrong parameter order