Array walk function in PHP

Introduction

The array walk function helps you change or use each item in a list one by one.

You want to add a fixed number to every score in a list.

You want to print each name in a list with a greeting.

You want to change all words in a list to uppercase.

You want to check or count items in a list based on a rule.

Syntax

PHP

<?php
function callback_function(&$value, $key, $user_data) {
    // Change or use $value here
}

array_walk(array &$array, callable $callback, mixed $user_data = null): bool;
?>

The callback function receives the value by reference, so you can change it directly.

The callback also gets the key and optional extra data you pass.

Examples

This adds 5 to each number in the list.

PHP

<?php
function add_five(&$value, $key) {
    $value += 5;
}

$numbers = [1, 2, 3];
array_walk($numbers, 'add_five');
print_r($numbers);

This prints a greeting for each name without changing the list.

PHP

<?php
function greet($value, $key) {
    echo "Hello, $value!\n";
}

$names = ['Anna', 'Bob'];
array_walk($names, 'greet');

This changes all words to uppercase.

PHP

<?php
function to_uppercase(&$value, $key) {
    $value = strtoupper($value);
}

$words = ['apple', 'banana'];
array_walk($words, 'to_uppercase');
print_r($words);

This adds ' fruit' to each word using extra data.

PHP

<?php
function add_suffix(&$value, $key, $suffix) {
    $value .= $suffix;
}

$fruits = ['apple', 'banana'];
array_walk($fruits, 'add_suffix', ' fruit');
print_r($fruits);

Sample Program

This program adds an exclamation mark to each word in the list and shows the list before and after.

PHP

<?php
function add_exclamation(&$value, $key) {
    $value .= '!';
}

$words = ['hello', 'world', 'php'];

// Before change
print_r($words);

// Change each word
array_walk($words, 'add_exclamation');

// After change
print_r($words);
?>

Important Notes

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

Space complexity is O(1) since it changes items in place.

Common mistake: forgetting to pass the value by reference in the callback, so changes don't apply.

Use array_walk when you want to change or use each item without creating a new list. Use array_map if you want a new list instead.

Summary

array_walk lets you run a function on each item in a list.

You can change items directly by passing them by reference.

You can also pass extra data to the function if needed.

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