Array reduce function in PHP - Step-by-Step Execution

Concept Flow - Array reduce function

Start with array

↓

Initialize accumulator with initial value

↓

Take first element

↓

Apply callback(accumulator, element)

↓

Update accumulator with callback result

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More elements?

Yes→Take next element

The reduce function starts with an initial value and applies a callback to combine each array element into a single result.

Execution Sample

PHP

<?php
$array = [1, 2, 3, 4];
$result = array_reduce($array, fn($acc, $item) => $acc + $item, 0);
echo $result;
?>

This code sums all numbers in the array using array_reduce starting from 0.

Execution Table

Step	Current Element	Accumulator Before	Callback Result	Accumulator After	Action
1	1	0	0 + 1 = 1	1	Add first element to accumulator
2	2	1	1 + 2 = 3	3	Add second element to accumulator
3	3	3	3 + 3 = 6	6	Add third element to accumulator
4	4	6	6 + 4 = 10	10	Add fourth element to accumulator
5	-	10	-	10	No more elements, return accumulator

💡 All elements processed, final accumulator 10 returned

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
accumulator	0	1	3	6	10	10
current element	-	1	2	3	4	-

Key Moments - 3 Insights

Why does the accumulator start with the initial value and not the first array element?

The accumulator starts with the initial value (0 here) so the callback always has a defined starting point. See execution_table row 1 where accumulator is 0 before adding the first element.

What happens if the array is empty?

If the array is empty, array_reduce returns the initial value immediately because there are no elements to process, as shown in the exit_note.

Why do we update the accumulator after each callback?

The accumulator holds the combined result so far. Each callback uses it to add the current element, then updates it for the next step, as shown in execution_table rows 1-4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the accumulator value after processing the second element?

A 2

B 3

C 1

D 4

💡 Hint

Check the 'Accumulator After' column at Step 2 in execution_table

At which step does the accumulator become 6?

A Step 1

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Accumulator After' column in execution_table for value 6

If the initial value was 10 instead of 0, what would be the final accumulator?

A 20

B 10

C 24

D 30

💡 Hint

Sum array elements 1+2+3+4=10 plus initial 10 equals 20, check calculation carefully

Concept Snapshot

array_reduce(array, callback, initial) applies callback to combine array elements into one value.
Starts with initial accumulator value.
Callback receives accumulator and current element.
Returns final accumulator after processing all elements.
Useful for sums, products, or custom reductions.

Full Transcript

The PHP array_reduce function takes an array, a callback function, and an initial value. It starts with the initial value as the accumulator. Then for each element in the array, it calls the callback with the accumulator and the element. The callback returns a new accumulator value. This repeats until all elements are processed. Finally, array_reduce returns the accumulator. For example, summing [1,2,3,4] with initial 0 results in 10. If the array is empty, it returns the initial value immediately. The accumulator updates after each element to hold the combined result so far.