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

Array reduce function

📖 Scenario: You are working on a simple shopping cart system. You have a list of prices for items a customer wants to buy.

🎯 Goal: You will use PHP's array_reduce function to calculate the total price of all items in the cart.

📋 What You'll Learn

Create an array called prices with the exact values: 10, 20, 30, 40

Create a variable called initial and set it to 0

Use array_reduce with a callback function named sum that adds two numbers

Store the result of array_reduce in a variable called total

Print the value of total

💡 Why This Matters

🌍 Real World

Calculating totals from lists of numbers is common in shopping carts, billing systems, and data analysis.

💼 Career

Understanding how to use array functions like <code>array_reduce</code> helps you write cleaner and more efficient PHP code in real projects.

1

Create the prices array

Create an array called prices with the exact values 10, 20, 30, and 40.

PHP

<?php

// Create the prices array with 10, 20, 30, 40
// Your code here

Hint

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

2

Set the initial value

Create a variable called initial and set it to 0.

PHP

<?php

$prices = [10, 20, 30, 40];

// Create the initial value variable and set it to 0
// Your code here

Hint

Just assign 0 to a variable named initial.

3

Use array_reduce to sum prices

Use array_reduce with a callback function named sum that takes two parameters and returns their sum. Store the result in a variable called total. Use $prices as the array and $initial as the initial value.

PHP

<?php

$prices = [10, 20, 30, 40];

$initial = 0;

// Use array_reduce with a callback function sum to add values
// Store the result in total
// Your code here

Hint

Define a function sum that adds two numbers. Then call array_reduce with $prices, the function name as a string, and $initial.

4

Print the total price

Print the value of the variable total.

PHP

<?php

$prices = [10, 20, 30, 40];

$initial = 0;

function sum($carry, $item) {
    return $carry + $item;
}

$total = array_reduce($prices, 'sum', $initial);

// Print the total
// Your code here

Hint

Use print($total); to show the total price.

Practice

1. What does the PHP function array_reduce do?

easy

A. It sorts the array elements in ascending order.

B. It combines all elements of an array into a single value using a callback function.

C. It filters elements of an array based on a condition.

D. It splits an array into smaller arrays.

Solution

1. Step 1: Understand the purpose of array_reduce

   The function takes an array and a callback to combine elements into one value.

2. Step 2: Compare with other array functions

   Unlike sorting or filtering, array_reduce accumulates values into one result.

3. Final Answer:

   It combines all elements of an array into a single value using a callback function. -> Option B

4. Quick Check:

   array_reduce = combine elements [OK]

Hint: Remember: array_reduce reduces array to one value [OK]

Common Mistakes:

• Confusing array_reduce with sorting functions
• Thinking it filters elements instead of combining
• Assuming it splits arrays into parts

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

easy

A. array_reduce(mixed $initial = null, array $array, callable $callback)

B. array_reduce(callable $callback, array $array, mixed $initial = null)

C. array_reduce(array $array, mixed $initial = null, callable $callback)

D. array_reduce(array $array, callable $callback, mixed $initial = null)

Solution

1. Step 1: Recall the official function signature

   The correct order is array, callback, then optional initial value.

2. Step 2: Check each option's parameter order

   Only array_reduce(array $array, callable $callback, mixed $initial = null) matches the correct order and types.

3. Final Answer:

   array_reduce(array $array, callable $callback, mixed $initial = null) -> Option D

4. Quick Check:

   array first, callback second, initial last [OK]

Hint: Remember: array first, callback second, initial last [OK]

Common Mistakes:

• Swapping callback and array parameters
• Placing initial value before callback
• Misordering parameters causing errors

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

$numbers = [1, 2, 3, 4];
$result = array_reduce($numbers, function($carry, $item) {
    return $carry + $item;
}, 0);
echo $result;

medium

A. 10

B. 24

C. 1234

D. Error

Solution

1. Step 1: Understand the callback function

   The callback adds the current item to the carry (accumulated sum).

2. Step 2: Calculate the sum step-by-step

   Starting with 0, add 1, then 2, then 3, then 4: 0+1=1, 1+2=3, 3+3=6, 6+4=10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Sum of 1+2+3+4 = 10 [OK]

Hint: Sum array with array_reduce and initial 0 [OK]

Common Mistakes:

• Forgetting initial value causes null carry
• Concatenating instead of adding numbers
• Expecting product instead of sum

4. Identify the issue in this PHP code using array_reduce:

$values = [];
$result = array_reduce($values, function($carry, $item) {
    return $carry + $item;
});
echo $result;

medium

A. Callback function has wrong parameter order.

B. array_reduce cannot be used on empty arrays.

C. Missing initial value causes $carry to be null.

D. The echo statement is missing parentheses.

Solution

1. Step 1: Check initial value usage

   No initial value is given, so $carry starts as null.

2. Step 2: Understand effect on empty array

   With empty array and no initial, array_reduce returns null without calling the callback.

3. Final Answer:

   Missing initial value causes $carry to be null. -> Option C

4. Quick Check:

   Initial value needed for empty arrays [OK]

Hint: Always provide initial value to avoid null carry [OK]

Common Mistakes:

• Assuming callback parameters are reversed
• Thinking array_reduce fails on empty arrays without initial
• Ignoring PHP syntax for echo

5. Given an array of strings ['apple', 'banana', 'cherry'], how can you use array_reduce to create a single string with all fruits separated by commas (e.g., 'apple,banana,cherry')?

hard

A. Use array_reduce with callback concatenating carry and item with a comma, initial value as empty string.

B. Use array_reduce with callback adding carry and item as numbers, initial value 0.

C. Use array_reduce with callback returning only the last item, no initial value.

D. Use array_reduce with callback multiplying carry and item, initial value 1.

Solution

1. Step 1: Understand the goal

   We want to join strings with commas into one string.

2. Step 2: Design the callback

   Callback should add a comma between carry and item, starting with empty string.

3. Step 3: Avoid leading comma

   Use a condition to avoid comma before first item or trim later.

4. Final Answer:

   Use array_reduce with callback concatenating carry and item with a comma, initial value as empty string. -> Option A

5. Quick Check:

   Concatenate strings with commas using array_reduce [OK]

Hint: Concatenate strings with commas, start with empty string [OK]

Common Mistakes:

• Adding strings as numbers causing errors
• Not providing initial value causing null carry
• Using multiplication on strings