Array filter function in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array_filter with callback

What is the output of the following PHP code?

PHP

<?php
$array = [1, 2, 3, 4, 5];
$result = array_filter($array, fn($x) => $x % 2 === 0);
print_r($result);
?>

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

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

C Array ( [0] => 1 [2] => 3 [4] => 5 )

D Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 4 [4] => 5 )

💡 Hint

Remember that array_filter preserves keys by default.

array_filter keeps elements where the callback returns true. Here, it keeps even numbers 2 and 4, preserving their original keys 1 and 3.

❓ Predict Output

intermediate

Effect of array_filter without callback

What will be the output of this PHP code?

PHP

<?php
$array = [0, 1, false, 2, '', 3];
$result = array_filter($array);
print_r($result);
?>

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

B Array ( [0] => 0 [1] => 1 [2] => false [3] => 2 [4] => '' [5] => 3 )

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

D Array ( [1] => 1 [3] => 2 [5] => 3 )

💡 Hint

Without a callback, array_filter removes values that are falsey.

array_filter without a callback removes falsey values like 0, false, and empty string, keeping only 1, 2, and 3 with their original keys.

🔧 Debug

advanced

Why does this array_filter code cause an error?

Consider this PHP code snippet. Why does it cause an error?

PHP

<?php
$array = [1, 2, 3, 4];
$result = array_filter($array, 'is_even');
print_r($result);

function is_even($n) {
  return $n % 2 == 0;
}
?>

A It runs correctly and outputs even numbers with keys preserved.

B It causes a syntax error because function names cannot be passed as strings.

C It causes a fatal error because the function is_even is called before it is defined.

D It causes a runtime error because array_filter expects an anonymous function.

💡 Hint

In PHP, functions must be defined before they are used if called by name as a string.

The function is_even is defined after it is used in array_filter, causing a fatal error because PHP does not find the function at the time of call.

📝 Syntax

advanced

Identify the syntax error in this array_filter usage

Which option shows the correct way to use array_filter with a callback that filters odd numbers?

A $result = array_filter($array, fn($x) => $x % 2 == 1);

B $result = array_filter($array, function($x) { return $x % 2 == 1; });

C $result = array_filter($array, function($x) { $x % 2 == 1 });

D $result = array_filter($array, fn $x => $x % 2 == 1);

💡 Hint

Anonymous functions need a return statement unless using arrow functions.

Option B correctly uses a function with a return statement. Option B is valid PHP 7.4+ arrow function syntax but uses == instead of === (still valid but less strict). Option B is invalid syntax. Option B misses return statement causing no filtering.

🚀 Application

expert

Count how many elements remain after filtering

Given the array and filter below, how many elements will remain in the result?

PHP

<?php
$array = ['apple', 'banana', 'cherry', 'date', 'fig'];
$result = array_filter($array, fn($fruit) => strlen($fruit) > 4);
$count = count($result);
echo $count;
?>

A 3

B 5

C 4

D 2

💡 Hint

Check the length of each fruit name carefully.

The fruits with length greater than 4 are 'apple' (5), 'banana' (6), and 'cherry' (6). 'date' (4) and 'fig' (3) are excluded. So 3 elements remain.