Array walk function in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code using array_walk?

Consider the following PHP code. What will it output when run?

PHP

<?php
$arr = [1, 2, 3];
array_walk($arr, function(&$value, $key) {
    $value *= 2;
});
print_r($arr);
?>

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

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

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

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

💡 Hint

Remember that array_walk passes values by reference if you want to modify the original array.

The function multiplies each element by 2 by reference, so the original array is changed.

❓ Predict Output

intermediate

What does this PHP code print?

Look at this PHP code using array_walk. What will be printed?

PHP

<?php
$arr = ['a' => 1, 'b' => 2];
array_walk($arr, function($value, $key) {
    echo "$key:$value ";
});
?>

A a:1 b:2

B 1:a 2:b

C a b

D Array

💡 Hint

The callback prints key and value pairs separated by a colon.

array_walk calls the callback for each element, printing key and value with a colon.

🔧 Debug

advanced

Why does this PHP code cause an error?

This PHP code tries to modify array elements using array_walk but causes an error. Why?

PHP

<?php
$arr = [1, 2, 3];
array_walk($arr, function($value) {
    $value *= 2;
});
print_r($arr);
?>

A Because array_walk requires the callback to return a value.

B Because array_walk cannot be used with numeric arrays.

C Because the callback parameter $value is not passed by reference, so original array is unchanged.

D Because print_r cannot print arrays modified by array_walk.

💡 Hint

Check how the callback parameters are declared and if they modify the original array.

Without passing $value by reference, changes inside the callback do not affect the original array.

📝 Syntax

advanced

Which option causes a syntax error in this PHP array_walk usage?

Identify which code snippet will cause a syntax error when using array_walk.

A array_walk($arr, function(&$v $k) { $v += 1; });

B array_walk($arr, function($v, $k) { $v += 1; });

C array_walk($arr, function(&$v, $k) { $v += 1; });

D array_walk($arr, fn(&$v) => $v += 1);

💡 Hint

Check the function parameter list syntax carefully.

Option A misses a comma between parameters, causing a syntax error.

🚀 Application

expert

How many items are in the array after this PHP code runs?

Given this PHP code using array_walk, how many items does the array have after execution?

PHP

<?php
$arr = [10, 20, 30];
array_walk($arr, function(&$v, $k) use (&$arr) {
    if ($v > 15) {
        unset($arr[$k]);
    }
});
print_r($arr);
?>

A 0

B 2

C 3

D 1

💡 Hint

Consider what happens when you unset elements during array_walk iteration.

Elements with values 20 and 30 are removed, leaving only the element with value 10.