Array filter function in PHP - Mini Project: Build & Apply

Array filter function

📖 Scenario: You work in a fruit store. You have a list of fruits with their prices. You want to find only the fruits that cost more than 2 dollars.

🎯 Goal: Build a PHP program that filters an array of fruits to keep only those with prices greater than 2 dollars using the array_filter function.

📋 What You'll Learn

Create an associative array called fruits with exact entries: 'apple' => 3, 'banana' => 1, 'cherry' => 5, 'date' => 2, 'elderberry' => 4

Create a variable called price_limit and set it to 2

Use array_filter with a callback function that keeps fruits with price greater than price_limit

Print the filtered array using print_r

💡 Why This Matters

🌍 Real World

Filtering arrays is common when you want to select only certain data from a list, like filtering products by price in an online store.

💼 Career

Understanding how to filter arrays helps in data processing tasks, backend development, and working with APIs that return lists of items.

1

Create the fruits array

Create an associative array called fruits with these exact entries: 'apple' => 3, 'banana' => 1, 'cherry' => 5, 'date' => 2, 'elderberry' => 4

PHP

<?php

// Create the fruits array with exact entries
// Your code here

Hint

Use square brackets [] to create the array and separate entries with commas.

2

Set the price limit

Create a variable called price_limit and set it to 2

PHP

 3, 'banana' => 1, 'cherry' => 5, 'date' => 2, 'elderberry' => 4];

// Create price_limit variable and set to 2
// Your code here

Hint

Use the = sign to assign the value 2 to price_limit.

3

Filter fruits by price

Use array_filter with a callback function that keeps fruits with price greater than price_limit. Store the result in a variable called expensive_fruits

PHP

 3, 'banana' => 1, 'cherry' => 5, 'date' => 2, 'elderberry' => 4];
$price_limit = 2;

// Use array_filter to keep fruits with price > price_limit
// Your code here

Hint

Use an anonymous function with use ($price_limit) to access the variable inside the callback.

4

Print the filtered fruits

Print the expensive_fruits array using print_r

PHP

 3, 'banana' => 1, 'cherry' => 5, 'date' => 2, 'elderberry' => 4];
$price_limit = 2;

$expensive_fruits = array_filter($fruits, function($price) use ($price_limit) {
    return $price > $price_limit;
});

// Print the expensive_fruits array
// Your code here

Hint

Use print_r($expensive_fruits); to display the filtered 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