Associative Arrays

An associative array uses meaningful keys — usually strings — instead of numbers: a product's name, price and stock; a user's settings; a word and how often it appears. In PHP, they're not a separate type: every array is a map from keys to values, and "indexed" arrays are simply ones whose keys happen to be 0, 1, 2…

Creating and reading

Use key => value pairs:

PHP 
<?php

$book = [
    "title" => "The Hobbit",
    "author" => "J. R. R. Tolkien",
    "year" => 1937,
    "inStock" => true,
];

echo $book["title"], " by ", $book["author"], "\n";
echo "Published {$book['year']}\n";

$book["price"] = 12.99;     // add a new key
$book["year"] = 1937;       // overwrite an existing one
echo count($book), " fields\n";
Output
The Hobbit by J. R. R. Tolkien
Published 1937
5 fields

The trailing comma after the last element is allowed and recommended: adding a line later then changes only one line in version control.

Keys

Keys can be integers or strings. PHP converts some key types automatically — numeric strings like "7" become integers, booleans become 0/1 — and a key can appear only once:

PHP 
<?php

$a = ["7" => "seven", 7 => "SEVEN", "07" => "zero-seven", true => "yes"];
var_dump($a);
Output
array(3) {
  [7]=>
  string(5) "SEVEN"
  ["07"]=>
  string(10) "zero-seven"
  [1]=>
  string(3) "yes"
}

Arrays remember the order in which keys were inserted, and foreach always follows that order.

Looping over keys and values

PHP 
<?php

$prices = ["coffee" => 3.20, "tea" => 2.50, "cake" => 4.75];

foreach ($prices as $item => $price) {
    printf("%-7s %5.2f\n", $item, $price);
}

echo implode(", ", array_keys($prices)), "\n";
echo array_sum(array_values($prices)), "\n";
Output
coffee   3.20
tea      2.50
cake     4.75
coffee, tea, cake
10.45

Checking for keys

PHP 
<?php

$stock = ["apples" => 12, "bananas" => 0, "cherries" => null];

var_dump(isset($stock["apples"]));
var_dump(isset($stock["cherries"]));            // null counts as not set
var_dump(array_key_exists("cherries", $stock)); // the key itself exists
echo $stock["pears"] ?? 0, "\n";
Output
bool(true)
bool(false)
bool(true)
0

isset is faster and usually what you want; array_key_exists distinguishes "missing" from "present but null".

Counting with an associative array

Using values as keys is the classic way to count occurrences or group things:

PHP 
<?php

$text = "the cat sat on the mat the end";
$counts = [];

foreach (explode(" ", $text) as $word) {
    $counts[$word] = ($counts[$word] ?? 0) + 1;
}
arsort($counts);    // sort by value, highest first, keeping keys

foreach ($counts as $word => $n) {
    echo str_pad($word, 4), str_repeat("#", $n), " $n\n";
}
print_r(array_count_values(["a", "b", "a", "c", "a"]));
Output
the ### 3
cat # 1
sat # 1
on  # 1
mat # 1
end # 1
Array
(
    [a] => 3
    [b] => 1
    [c] => 1
)

array_count_values does the counting in one call. Sorting functions like arsort are covered in lesson 24.

Grouping

PHP 
<?php

$people = [
    ["name" => "Amir", "city" => "Leeds"],
    ["name" => "Bea", "city" => "York"],
    ["name" => "Cleo", "city" => "Leeds"],
];

$byCity = [];
foreach ($people as $person) {
    $byCity[$person["city"]][] = $person["name"];
}

foreach ($byCity as $city => $names) {
    echo "$city: ", implode(", ", $names), "\n";
}
Output
Leeds: Amir, Cleo
York: Bea

$byCity[$city][] = … creates the inner array automatically the first time a city appears.

Arrays as records

Associative arrays are often used as simple records — rows from a database or decoded JSON usually arrive this way. For data with a fixed shape that you pass around a lot, a class with typed properties (lesson 33) gives you autocompletion and error checking; arrays are best for quick, local or truly dynamic data.

Combining associative arrays

With string keys, array_merge and the + operator behave differently: array_merge lets later values win, while + keeps the first value for each key — handy for applying defaults:

PHP 
<?php

$defaults = ["theme" => "light", "fontSize" => 14, "lang" => "en"];
$user = ["theme" => "dark", "fontSize" => 16];

print_r(array_merge($defaults, $user));
print_r($user + $defaults);
print_r(array_flip(["red" => 1, "green" => 2]));
Output
Array
(
    [theme] => dark
    [fontSize] => 16
    [lang] => en
)
Array
(
    [theme] => dark
    [fontSize] => 16
    [lang] => en
)
Array
(
    [1] => red
    [2] => green
)

Careful: with numeric keys, array_merge appends and renumbers, while + keeps the left-hand values for duplicate indexes.

Try it yourself

Scan items at a shop till: count the quantities and price them from a price list, skipping unknown items.

Try it yourself · main.php
Edit the code, copy it into main.php, then run it with php main.php (PHP 8.5). How to install PHP
Output
apple   x3   1.20
milk    x1   1.15
bread   x1   2.10
cheese  x1  (no price)
Total: 4.45
PreviousArrays