16. PHP Associative Arrays

What you will learn

What an associative array is
How to read, add, and remove keys
How to loop with keys and values
How to sort, check for keys, and nest arrays

Named keys

An associative array uses names (keys) instead of numbers. It stores data in key and value pairs, such as a person, a product, or a form submission. The arrow => links a key to its value.

PHP code

<?php
$person = [
    "name" => "Sam",
    "age" => 25,
    "city" => "Lahore"
];

echo $person["name"] . " is " . $person["age"] . " and lives in " . $person["city"] . ".";

Output

Sam is 25 and lives in Lahore.

Changing, adding, and removing

PHP code

<?php
$car = ["brand" => "Toyota", "year" => 2020];

$car["year"] = 2024;
$car["color"] = "blue";
unset($car["brand"]);

echo implode(", ", array_keys($car)) . " | color: " . $car["color"];

Output

year, color | color: blue

Looping with keys and values

PHP code

<?php
$lesson = ["title" => "Arrays", "order" => 16, "free" => true];

foreach ($lesson as $key => $value) {
    echo $key . " = " . var_export($value, true) . "<br>";
}

What the browser shows

title = ‘Arrays’
order = 16
free = true

var_export() prints a value as PHP code, which is why the text appears in quotes.

Checking for keys safely

Reading a key that does not exist gives a warning. Check first, or use ??.

PHP code

<?php
$user = ["name" => "Sam", "phone" => null];

var_dump(array_key_exists("phone", $user));
var_dump(isset($user["phone"]));
echo $user["email"] ?? "no email";

What you should see

bool(true)
bool(false)
no email

array_key_exists() is true even when the value is null, while isset() is false for null.

Keys and values

PHP code

<?php
$stock = ["pens" => 40, "books" => 12, "bags" => 5];

echo implode(", ", array_keys($stock)) . "<br>";
echo implode(", ", array_values($stock)) . "<br>";
echo "Total items: " . array_sum($stock);

What the browser shows

pens, books, bags
40, 12, 5
Total items: 57

Sorting by key or value

Function	Sorts by	Keeps the key link?
`asort($a)`	Value, low to high	Yes
`arsort($a)`	Value, high to low	Yes
`ksort($a)`	Key, A to Z	Yes
`krsort($a)`	Key, Z to A	Yes

PHP code

<?php
$scores = ["Sam" => 72, "Ali" => 90, "Mia" => 85];

arsort($scores);
foreach ($scores as $name => $score) {
    echo "$name: $score<br>";
}

What the browser shows

Ali: 90
Mia: 85
Sam: 72

Plain sort() would throw the keys away, so use the a and k versions for associative arrays.

Arrays of associative arrays

Real data is often a list of records.

PHP code

<?php
$products = [
    ["name" => "Notebook", "price" => 4.5],
    ["name" => "Pen", "price" => 1.2],
    ["name" => "Bag", "price" => 18]
];

$total = 0;
foreach ($products as $product) {
    echo htmlspecialchars($product["name"]) . ": " . number_format($product["price"], 2) . "<br>";
    $total += $product["price"];
}
echo "Total: " . number_format($total, 2);

What the browser shows

Notebook: 4.50
Pen: 1.20
Bag: 18.00
Total: 23.70

array_column($products, "name") pulls one column out as a plain list.

JSON and arrays

PHP converts between arrays and JSON text, which is how data moves to and from JavaScript.

PHP code

<?php
$data = ["name" => "Sam", "skills" => ["HTML", "PHP"]];

$json = json_encode($data);
echo $json . "<br>";

$back = json_decode($json, true);
echo $back["skills"][1];

What the browser shows

{“name”:”Sam”,”skills”:[“HTML”,”PHP”]}
PHP

Pass true to json_decode() to get an associative array instead of an object.

Common mistakes

Reading a key that does not exist, without ?? or isset().
Misspelling a key, since keys are case sensitive.
Using sort() on an associative array and losing the keys.
Forgetting the => arrow, or the comma between items.
Printing values from outside without htmlspecialchars().

Exercise

Create $book with the keys title, author, and pages (320). Print Title by Author (320 pages) using its values.

Show solution

<?php
$book = [
    "title" => "Learning Code",
    "author" => "Sam Rivera",
    "pages" => 320
];

echo $book["title"] . " by " . $book["author"] . " (" . $book["pages"] . " pages)";

Knowledge check

1. Which symbol links a key to its value?

The arrow =>.

2. Which function sorts by value but keeps the keys?

asort() (or arsort() for descending).

3. Which function turns an array into JSON text?

json_encode()

Summary

Associative arrays store key and value pairs.
Loop with foreach ($a as $key => $value), and check keys with ?? or isset().
Use asort and ksort to sort, and json_encode and json_decode to exchange data.