Array filter function in PHP

Introduction

The array filter function helps you pick only the items you want from a list. It makes your list smaller by keeping items that match a rule.

You have a list of numbers and want only the even ones.

You want to remove empty or null values from a list of strings.

You want to find all users older than 18 from a list of users.

You want to keep only the words that start with a certain letter.

Syntax

PHP

<?php
function array_filter(array $array, callable $callback): array
{
    $filteredArray = [];
    foreach ($array as $key => $value) {
        if ($callback($value, $key)) {
            $filteredArray[$key] = $value;
        }
    }
    return $filteredArray;
}
?>

The callback function decides if an item stays. It returns true to keep, false to remove.

The original keys are kept in the filtered array.

Examples

Filters only even numbers from the list.

PHP

<?php
$numbers = [1, 2, 3, 4, 5];
$evenNumbers = array_filter($numbers, fn($num) => $num % 2 === 0);
print_r($evenNumbers);

Filtering an empty array returns an empty array.

PHP

<?php
$emptyArray = [];
$filteredEmpty = array_filter($emptyArray, fn($item) => $item > 0);
print_r($filteredEmpty);

Filtering a single-item array that matches the condition keeps that item.

PHP

<?php
$singleItem = [10];
$filteredSingle = array_filter($singleItem, fn($item) => $item > 5);
print_r($filteredSingle);

Filters words starting with 'a'.

PHP

<?php
$words = ['apple', 'banana', 'avocado'];
$filteredWords = array_filter($words, fn($word) => str_starts_with($word, 'a'));
print_r($filteredWords);

Sample Program

This program shows the original list of numbers, then filters to keep only even numbers, and prints the result.

PHP

<?php
// Original list of numbers
$numbers = [1, 2, 3, 4, 5, 6];

// Print original list
echo "Original numbers:\n";
print_r($numbers);

// Filter function to keep only even numbers
$evenNumbers = array_filter($numbers, function($number) {
    return $number % 2 === 0;
});

// Print filtered list
echo "\nFiltered even numbers:\n";
print_r($evenNumbers);
?>

Important Notes

Time complexity is O(n) because it checks each item once.

Space complexity is O(n) in the worst case if all items pass the filter.

Common mistake: forgetting the callback must return true or false.

Use array_filter when you want to keep items matching a condition. Use array_map if you want to change items instead.

Summary

array_filter keeps only items that match a rule you give.

The callback function returns true to keep an item, false to remove it.

It keeps the original keys of the array.

Practice

1. What does the PHP function array_filter do to an array?

easy

A. It removes all keys from the array.

B. It keeps only the elements where the callback returns true.

C. It changes all values to strings.

D. It sorts the array in ascending order.

Solution

1. Step 1: Understand the purpose of array_filter

   The function array_filter tests each element with a callback function.

2. Step 2: Determine what happens based on callback result

   Elements for which the callback returns true are kept; others are removed.

3. Final Answer:

   It keeps only the elements where the callback returns true. -> Option B

4. Quick Check:

   array_filter keeps elements with true callback [OK]

Hint: Remember: true keeps, false removes elements [OK]

Common Mistakes:

• Thinking array_filter sorts the array
• Assuming it changes values to strings
• Believing it removes keys

2. Which of the following is the correct syntax to use array_filter with a callback function named isPositive on array $numbers?

easy

A. array_filter($numbers, isPositive);

B. array_filter($numbers, &isPositive);

C. array_filter(isPositive, $numbers);

D. array_filter($numbers, 'isPositive');

Solution

1. Step 1: Recall array_filter syntax

   The second argument must be a callable, passed as a string or anonymous function.

2. Step 2: Identify correct callable format

   Passing the function name as a string, like 'isPositive', is correct.

3. Final Answer:

   array_filter($numbers, 'isPositive'); -> Option D

4. Quick Check:

   Callback function name as string [OK]

Hint: Callback function names go in quotes [OK]

Common Mistakes:

• Passing callback without quotes
• Swapping argument order
• Using & before function name

3. What is the output of the following PHP code?

$arr = [1, 2, 3, 4, 5];
$result = array_filter($arr, fn($n) => $n % 2 === 0);
print_r($result);

medium

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

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

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

D. Array ( [0] => 2 [2] => 4 )

Solution

1. Step 1: Understand the callback function

   The callback keeps numbers where $n % 2 === 0, meaning even numbers.

2. Step 2: Identify even numbers and keys

   Even numbers in the array are 2 and 4 at keys 1 and 3 respectively.

3. Final Answer:

   Array ( [1] => 2 [3] => 4 ) -> Option C

4. Quick Check:

   Only even numbers kept with original keys [OK]

Hint: Check which elements return true, keys stay same [OK]

Common Mistakes:

• Assuming keys are reindexed
• Keeping odd numbers by mistake
• Confusing array keys with values

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

$data = [10, 15, 20];
$result = array_filter($data, function($x) {
  if ($x > 15) return true;
});
print_r($result);

medium

A. The callback function does not return false for some values.

B. array_filter requires a third argument for keys.

C. The array must be associative for array_filter to work.

D. The print_r function cannot print arrays.

Solution

1. Step 1: Check callback return values

   The callback returns true only if $x > 15, but returns nothing (null) otherwise.

2. Step 2: Understand array_filter behavior

   Callback must return true or false explicitly; null is treated as false but is unclear and risky.

3. Final Answer:

   The callback function does not return false for some values. -> Option A

4. Quick Check:

   Callback must return true or false explicitly [OK]

Hint: Always return true or false in callback [OK]

Common Mistakes:

• Missing explicit false return
• Thinking array_filter needs 3 arguments
• Believing print_r can't print arrays

5. Given an array $items = ['apple' => 3, 'banana' => 0, 'cherry' => 5, 'date' => null];, which array_filter call will keep only items with values that are positive numbers (greater than zero)?

hard

A. array_filter($items, fn($v) => $v > 0);

B. array_filter($items, fn($v) => (bool)$v);

C. array_filter($items);

D. array_filter($items, fn($v) => $v !== 0);

Solution

1. Step 1: Understand the goal

   We want to keep only values greater than zero (positive numbers).

2. Step 2: Analyze each option

   array_filter($items, fn($v) => $v > 0); keeps values where $v > 0, exactly what we want.
   array_filter($items, fn($v) => (bool)$v); keeps all truthy values, but 0 is false and null is false, so it removes them but also keeps negative numbers if any.
   array_filter($items); uses default filter which removes falsy values (0, null, false), but does not check for > 0.
   array_filter($items, fn($v) => $v !== 0); removes only zeros but keeps null and negative numbers.

3. Final Answer:

   array_filter($items, fn($v) => $v > 0); -> Option A

4. Quick Check:

   Filter with $v > 0 keeps only positive numbers [OK]

Hint: Use explicit condition for positive numbers [OK]

Common Mistakes:

• Using default filter which keeps all truthy values
• Filtering only non-zero but allowing null
• Confusing truthy with positive