Associative Arrays

A school locker room doesn't number the lockers 0, 1, 2. Each one has a name tag: "Maria", "Ben", "Carlo". You find a locker by its name.

In Indexed Arrays, every value had a number. That's perfect for a list of similar things. But a student's record has different kinds of information: a name, an age, a section. Numbers like $student[0] and $student[2] would be confusing. What's at index 2 again?

An associative array uses names as its keys instead.

Name tags instead of numbers

<?php
$student = [
    "name" => "Maria Santos",
    "age" => 16,
    "section" => "Rizal",
];

echo $student["name"], "\n";
echo $student["name"] . " is in section " . $student["section"] . ".\n";

Maria Santos
Maria Santos is in section Rizal.

• Each entry is key => value. The => arrow links a key to its value.
• The keys are usually strings, in quotes.
• You read a value with its key in square brackets: $student["name"].
• The comma after the last entry is optional, but it's good style: adding a line later won't need you to fix the line above.

Each key can appear only once. Keys are like labels on lockers: two lockers can't have the same name tag.

Inside double quotes

Putting an array value in a double-quoted string needs curly braces around it:

<?php
$student = ["name" => "Maria", "age" => 16];
echo "{$student["name"]} is {$student["age"]}.\n";

Maria is 16.

Or use the dot, which many people find clearer for array values.

Adding, changing, and removing

<?php
$prices = ["turon" => 25, "banana cue" => 20];

$prices["turon"] = 30;
$prices["kwek-kwek"] = 15;
unset($prices["banana cue"]);

print_r($prices);

Array
(
    [turon] => 30
    [kwek-kwek] => 15
)

• Assigning to an existing key changes its value.
• Assigning to a new key adds an entry.
• unset(...) removes an entry.

Is the key there?

Reading a key that doesn't exist gives a warning:

<?php
$student = ["name" => "Maria", "age" => 16];
echo $student["email"];
// error: Warning: Undefined array key "email"

There are three good ways to check first:

<?php
$student = ["name" => "Maria", "age" => 16, "club" => null];

var_dump(isset($student["name"]));
var_dump(isset($student["club"]));
var_dump(array_key_exists("club", $student));
echo $student["email"] ?? "no email", "\n";

bool(true)
bool(false)
bool(true)
no email

• isset is true if the key exists and its value isn't null.
• array_key_exists is true if the key exists at all, even if its value is null.
• ?? (from Operators and Expressions) gives a default when the key is missing, without a warning. It's the one you'll use most.

Looping over keys and values

foreach with $key => $value gives you both:

<?php
$stock = ["pencils" => 40, "erasers" => 12, "rulers" => 0];

foreach ($stock as $item => $count) {
    echo "$item: $count\n";
}

pencils: 40
erasers: 12
rulers: 0

Items come out in the order they were added. You can also get just the keys with array_keys(), or just the values with array_values().

Arrays inside arrays

The real power comes from putting arrays inside arrays. A list of students, where each student is an associative array, is how PHP programs usually hold table-like data, such as rows from a database:

<?php
$students = [
    ["name" => "Maria", "grade" => 92],
    ["name" => "Ben", "grade" => 85],
    ["name" => "Carlo", "grade" => 78],
];

echo $students[1]["name"], "\n";

foreach ($students as $student) {
    echo $student["name"] . ": " . $student["grade"] . "\n";
}

Ben
Maria: 92
Ben: 85
Carlo: 78

$students[1] is Ben's whole record, and $students[1]["name"] reaches inside it. Read it from left to right: "the second student, then their name."

Try it

A shop totals up an order. Predict each line.

Running PHP here

Running PHP right in the browser is coming to Syntaxia soon. For now, you can read and edit the code here. If you have PHP installed on your computer (see Setting Up), save it as index.php and run it with php index.php.

Check your prediction

2 x adobo = 240
3 x rice = 75
Sorry, no halo-halo today.
Total: 315
Dishes on the menu: adobo, sinigang, rice

Rice went up to 25 before the loop, so 3 rice cost 75. Halo-halo isn't a key in $menu, so isset is false, and continue skips it. array_keys lists the dish names in the order they were added.

Try it yourself

1. Add "halo-halo" => 90 to the menu. What's the new total?
2. Add a "note" key to one order line, and print it with ?? so lines without a note print (no note).
3. Make an associative array of 3 friends and their birthdays, and loop over it to print Name: birthday for each.

Check your understanding

How do you read the age from $person = ["name" => "Ana", "age" => 15]?

$person["age"]

$person[1]

$person->age

$person.age

Which gives a default value, with no warning, when a key is missing?

$data["key"] || "default"

unset($data["key"])

$data["key"] ?? "default"

count($data)

In $students[2]["name"], what does the [2] pick?

The second student

The third letter of the name

The key called 2 inside the name

The third student's whole record

Up next

You've been looping by hand to find, total, and filter. PHP has built-in functions that do most of that for you, in one line. Meet them in Array Functions: sort, filter, and map.