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

Array walk function

📖 Scenario: You have a list of fruits with their prices. You want to increase each fruit's price by 10% to prepare for a sale.

🎯 Goal: Use the PHP array_walk function to update each fruit's price by increasing it by 10%.

📋 What You'll Learn

Create an associative array called fruits with exact keys and values

Create a function called increasePrice that increases the price by 10%

Use array_walk with increasePrice to update the prices

Print the updated fruits array

💡 Why This Matters

🌍 Real World

Increasing prices or applying discounts to product lists is common in online stores and inventory management.

💼 Career

Understanding how to manipulate arrays with functions like <code>array_walk</code> is useful for backend PHP developers working with data transformations.

1

Create the fruits array

Create an associative array called fruits with these exact entries: 'apple' => 100, 'banana' => 50, 'cherry' => 75.

PHP

<?php

// Create the fruits array with exact keys and values
// Your code here

Hint

Use square brackets [] to create the array with keys and values.

2

Create the increasePrice function

Create a function called increasePrice that takes two parameters: &$price and $key. Inside the function, increase $price by 10% using $price *= 1.1;.

PHP

 100, 'banana' => 50, 'cherry' => 75];

// Create the increasePrice function below
// Your code here

Hint

Remember to pass $price by reference using & so the original array updates.

3

Use array_walk to update prices

Use array_walk with the $fruits array and the increasePrice function to update each fruit's price.

PHP

 100, 'banana' => 50, 'cherry' => 75];

function increasePrice(&$price, $key) {
    $price *= 1.1;
}

// Use array_walk to apply increasePrice to $fruits
// Your code here

Hint

Use the function name as a string in array_walk.

4

Print the updated fruits array

Print the $fruits array using print_r($fruits); to show the updated prices.

PHP

 100, 'banana' => 50, 'cherry' => 75];

function increasePrice(&$price, $key) {
    $price *= 1.1;
}

array_walk($fruits, 'increasePrice');

// Print the updated fruits array
// Your code here

Hint

Use print_r to display the array in a readable format.

Practice

1. What does the PHP function array_walk do?

easy

A. It applies a user-defined function to every element of an array.

B. It sorts the array in ascending order.

C. It merges two arrays into one.

D. It removes duplicate values from an array.

Solution

1. Step 1: Understand the purpose of array_walk

   The function array_walk is designed to apply a callback function to each element of an array.

2. Step 2: Compare with other array functions

   Sorting, merging, or removing duplicates are done by other functions like sort, array_merge, or array_unique.

3. Final Answer:

   It applies a user-defined function to every element of an array. -> Option A

4. Quick Check:

   array_walk = apply function to each element [OK]

Hint: array_walk runs a function on each array item [OK]

Common Mistakes:

• Confusing array_walk with sorting functions
• Thinking it merges arrays
• Assuming it removes duplicates

2. Which of the following is the correct syntax to use array_walk with a callback function named modify on an array $arr?

easy

A. array_walk('modify', $arr);

B. array_walk(modify, $arr);

C. array_walk($arr, modify);

D. array_walk($arr, 'modify');

Solution

1. Step 1: Recall array_walk syntax

   The correct syntax is array_walk(array, callback), where callback is a string with the function name.

2. Step 2: Check the options

   Only array_walk($arr, 'modify'); matches the correct order and uses quotes around the callback function name.

3. Final Answer:

   array_walk($arr, 'modify'); -> Option D

4. Quick Check:

   array_walk(array, 'function') syntax [OK]

Hint: array_walk takes array first, then callback as string [OK]

Common Mistakes:

• Swapping the order of parameters
• Omitting quotes around callback name
• Passing callback without quotes

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

$numbers = [1, 2, 3];
array_walk($numbers, function(&$value) {
    $value *= 2;
});
print_r($numbers);

medium

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

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

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

D. Error: Cannot modify array elements

Solution

1. Step 1: Understand the callback function

   The callback multiplies each value by 2, and the value is passed by reference (&$value), so changes affect the original array.

2. Step 2: Apply changes to each element

   1*2=2, 2*2=4, 3*2=6, so the array becomes [2, 4, 6].

3. Final Answer:

   Array ( [0] => 2 [1] => 4 [2] => 6 ) -> Option A

4. Quick Check:

   Reference callback doubles values [OK]

Hint: Pass value by reference to modify array items [OK]

Common Mistakes:

• Not using & to modify original array
• Expecting original array unchanged
• Confusing output format

4. Identify the error in this code snippet:

$arr = [1, 2, 3];
array_walk($arr, function($val) {
    $val += 5;
});
print_r($arr);

medium

A. The array must be passed by reference to array_walk.

B. array_walk requires a named function, not an anonymous function.

C. Callback function parameter should be passed by reference to modify array.

D. print_r cannot print arrays modified by array_walk.

Solution

1. Step 1: Check how callback modifies array

   The callback parameter $val is passed by value, so changes inside do not affect original array.

2. Step 2: Correct usage for modification

   Passing &$val by reference allows modifying the original array elements.

3. Final Answer:

   Callback function parameter should be passed by reference to modify array. -> Option C

4. Quick Check:

   Use & in callback to change array items [OK]

Hint: Use & in callback parameter to modify array elements [OK]

Common Mistakes:

• Ignoring reference in callback parameter
• Thinking array_walk needs array by reference
• Believing anonymous functions are not allowed

5. Given the array $fruits = ['apple' => 2, 'banana' => 3, 'cherry' => 5];, how can you use array_walk to add 10 to each value only if the value is less than 4?

hard

A. array_walk($fruits, function($val) { if ($val < 4) $val += 10; });

B. array_walk($fruits, function(&$val) { if ($val < 4) $val += 10; });

C. array_walk($fruits, function(&$val, $key) { if ($key < 4) $val += 10; });

D. array_walk($fruits, function(&$val, $key) { if ($val > 4) $val += 10; });

Solution

1. Step 1: Identify condition and modification

   We want to add 10 to values less than 4, so check $val < 4 inside the callback.

2. Step 2: Ensure modification affects original array

   Pass $val by reference (&$val) to change original array values.

3. Step 3: Verify options

   array_walk($fruits, function(&$val) { if ($val < 4) $val += 10; }); correctly uses reference and condition. array_walk($fruits, function($val) { if ($val < 4) $val += 10; }); lacks reference, so no change. array_walk($fruits, function(&$val, $key) { if ($key < 4) $val += 10; }); checks key instead of value. array_walk($fruits, function(&$val, $key) { if ($val > 4) $val += 10; }); adds 10 when value is greater than 4, which is wrong.

4. Final Answer:

   array_walk($fruits, function(&$val) { if ($val < 4) $val += 10; }); -> Option B

5. Quick Check:

   Reference + condition on value < 4 [OK]

Hint: Use & and if condition inside callback to selectively modify [OK]

Common Mistakes:

• Not passing value by reference
• Checking key instead of value
• Using wrong comparison operator