How do arrays work in PHP?

Question

Accepted Answer

PHP's **array** is an extraordinarily versatile, ordered map — it serves as both an **indexed list** and an **associative array (key-value map)**, and it's PHP's most-used data structure. A single array type covers what other languages split into lists, maps, and more.

## Indexed and associative arrays

```php
<?php
// indexed array (list)
$fruits = ["apple", "banana", "cherry"];
$fruits[0];              // "apple"
$fruits[] = "date";       // append

// associative array (key → value map)
$user = ["name" => "Ann", "age" => 30];
$user["name"];            // "Ann"
$user["email"] = "a@x.com";   // add a key

// they're the SAME type — keys can be int or string, mixed
$mixed = [0 => "a", "key" => "b"];
```

Both list-style and map-style arrays are the same `array` type — keys can be integers or strings.

## Iterating

```php
foreach ($fruits as $fruit) { echo $fruit; }              // values
foreach ($user as $key => $value) { echo "$key: $value"; }  // keys + values
```

`foreach` is the idiomatic way to loop arrays, with an optional `$key => $value` form for associative arrays.

## The rich array function library

```php
count($fruits);                              // size
in_array("apple", $fruits);                  // membership check
array_keys($user); array_values($user);       // extract keys / values
sort($fruits);                                // sort (in place)
array_merge($a, $b);                          // combine

// functional helpers
array_map(fn($x) => $x * 2, [1, 2, 3]);       // [2, 4, 6] — transform
array_filter([1, 2, 3, 4], fn($x) => $x % 2); // keep matching
array_reduce([1, 2, 3], fn($c, $x) => $c + $x, 0);  // 6 — fold
```

PHP has a vast library of `array_*` functions — `array_map`/`array_filter`/`array_reduce` for functional transforms, plus dozens more (sort, search, slice, merge).

## Multidimensional arrays

```php
$matrix = [[1, 2], [3, 4]];
$matrix[0][1];   // 2
$users = [["name" => "Ann"], ["name" => "Bob"]];   // array of associative arrays (common)
```

## Why it matters

Arrays are central to PHP — they're the language's all-purpose data structure, used constantly for lists, maps, configuration, database rows, and structured data, so mastering them is essential everyday knowledge.

The key insight is PHP's unusual design: a **single array type serves as both an indexed list and an associative map** (unlike languages that separate these), making it remarkably flexible.

Knowing how to create and access both forms, iterate with `foreach` (including the `$key => $value` form), and especially leverage the **rich library of `array_*` functions** (particularly `array_map`/`array_filter`/`array_reduce` for clean functional-style transformations) is what makes you productive in PHP, since so much PHP code revolves around manipulating arrays.

Arrays appear everywhere — query results, form data, API payloads, configuration — making fluency with them and their extensive function library fundamental to writing effective, idiomatic PHP.