Array map function in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the array_map function do in PHP?

It applies a callback function to each element of one or more arrays and returns a new array with the modified elements.

beginner

How do you pass a function to array_map?

You pass the function name as a string or an anonymous function as the first argument to array_map.

intermediate

What happens if you pass multiple arrays to array_map?

The callback function receives elements from each array as arguments, and array_map processes elements with the same keys together.

beginner

Example: What is the output of this code?

$numbers = [1, 2, 3];
$result = array_map(fn($n) => $n * 2, $numbers);
print_r($result);

The output is:

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

Each number is doubled by the callback function.

intermediate

Why is array_map useful compared to a foreach loop?

array_map is more concise and expressive for transforming arrays, making code easier to read and maintain.

What is the first argument of array_map in PHP?

A A callback function

B An array to modify

C A string to append

D A number to multiply

The first argument must be a callback function that defines how to transform each element.

What does array_map return?

A An integer count

B The original array unchanged

C A boolean value

D A new array with transformed elements

array_map returns a new array with each element modified by the callback.

If you pass two arrays to array_map, how many arguments does the callback receive?

A One argument

B Two arguments

C Zero arguments

D Three arguments

The callback receives one element from each array as separate arguments.

Which of these is a valid way to use array_map?

A array_map('strtoupper', ['a', 'b', 'c']);

B array_map(['a', 'b', 'c'], 'strtoupper');

C array_map('strtoupper');

D array_map();

The first argument is the callback function name, the second is the array.

What happens if the arrays passed to array_map have different lengths?

A An error is thrown

B The callback runs until the longest array ends

C The callback runs until the shortest array ends

D Only the first array is processed

Processing stops when the shortest array runs out of elements.

Explain how the array_map function works in PHP and give a simple example.

Think about how you change each item in a list using a function.

Describe what happens when you pass multiple arrays to array_map and how the callback function handles them.

Imagine pairing items from two lists and processing them together.

Practice

1. What does the PHP function array_map do?

easy

A. Deletes elements from an array based on a condition

B. Merges two or more arrays into one

C. Sorts the elements of an array in ascending order

D. Applies a function to each element of an array and returns a new array

Solution

1. Step 1: Understand the purpose of array_map

   The function applies a given callback function to each element of the input array.

2. Step 2: Identify the output of array_map

   It returns a new array with the results of the callback function applied to each element.

3. Final Answer:

   Applies a function to each element of an array and returns a new array -> Option D

4. Quick Check:

   array_map = apply function to array elements [OK]

Hint: Remember: array_map transforms each item, returns new array [OK]

Common Mistakes:

• Thinking array_map modifies the original array
• Confusing array_map with array_filter
• Assuming array_map merges arrays

2. Which of the following is the correct syntax to use array_map in PHP?

easy

A. array_map(function, array);

B. array_map(array, function);

C. array_map->function(array);

D. array_map(function => array);

Solution

1. Step 1: Recall the function signature

   The correct syntax is array_map(callback_function, array_to_process).

2. Step 2: Match the options with the syntax

   array_map(function, array); matches the correct order: function first, then array.

3. Final Answer:

   array_map(function, array); -> Option A

4. Quick Check:

   array_map(callback, array) syntax [OK]

Hint: Function comes first, array second in array_map [OK]

Common Mistakes:

• Swapping the order of function and array
• Using arrow or assignment syntax incorrectly
• Using object method syntax instead of function call

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

$numbers = [1, 2, 3];
$result = array_map(fn($n) => $n * 2, $numbers);
print_r($result);

medium

A. [2, 4, 6]

B. [1, 2, 3]

C. [3, 4, 5]

D. Error: Invalid callback

Solution

1. Step 1: Understand the callback function

   The callback doubles each number: $n * 2.

2. Step 2: Apply the callback to each element

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

3. Final Answer:

   [2, 4, 6] -> Option A

4. Quick Check:

   Each element doubled = [2, 4, 6] [OK]

Hint: Multiply each element by 2 in the callback [OK]

Common Mistakes:

• Expecting original array unchanged
• Confusing the callback operation
• Thinking print_r outputs a string, not array

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

$arr = [1, 2, 3];
$result = array_map('double', $arr);
function double($x) {
  return $x * 2;
}
print_r($result);

medium

A. array_map requires an anonymous function, not a named function

B. No error, code runs correctly

C. Function double is called before it is defined

D. Missing semicolon after array_map call

Solution

1. Step 1: Check function definition and usage

   The function double is defined after the array_map call, but PHP allows this because functions are hoisted.

2. Step 2: Verify syntax and function type

   array_map accepts named functions as callbacks, and semicolons are correctly placed.

3. Final Answer:

   No error, code runs correctly -> Option B

4. Quick Check:

   Named function callback works fine [OK]

Hint: PHP functions can be used before definition [OK]

Common Mistakes:

• Assuming function must be defined before use
• Thinking only anonymous functions work in array_map
• Missing semicolon errors where none exist

5. Given an array of strings ['apple', 'banana', 'cherry'], how can you use array_map to create a new array with the length of each string?

hard

A. array_map(fn($s) => size($s), ['apple', 'banana', 'cherry']);

B. array_map(fn($s) => count($s), ['apple', 'banana', 'cherry']);

C. array_map(fn($s) => strlen($s), ['apple', 'banana', 'cherry']);

D. array_map(fn($s) => $s.length, ['apple', 'banana', 'cherry']);

Solution

1. Step 1: Identify the correct function to get string length

   In PHP, strlen() returns the length of a string.

2. Step 2: Apply strlen in array_map callback

   Using fn($s) => strlen($s) applies length calculation to each string.

3. Final Answer:

   array_map(fn($s) => strlen($s), ['apple', 'banana', 'cherry']); -> Option C

4. Quick Check:

   Use strlen() for string length in array_map [OK]

Hint: Use strlen() inside array_map callback for string lengths [OK]

Common Mistakes:

• Using count() which counts array elements, not string length
• Trying to access string length with dot notation
• Using non-existent size() function