PHP Zero to Advanced #5: Arrays & Array Functions

Arrays are the workhorse of PHP. A single PHP array can act as a list, a dictionary (map), a stack, a queue, or a nested tree of data. Form submissions, database rows, JSON payloads, and configuration files all end up as arrays. In this part you will learn how to create and manipulate arrays, and how to use the built-in functions that save you from writing loops by hand.

Indexed Arrays

An indexed array uses automatic integer keys starting at 0. The short syntax [] is preferred over the older array().

<?php

$colors = ["red", "green", "blue"];

echo $colors[0] . "\n";
echo count($colors) . "\n";

$colors[] = "yellow";        // append
$colors[1] = "lime";         // update
print_r($colors);

echo $colors[count($colors) - 1] . "\n";   // last element

Output:

red
3
Array
(
    [0] => red
    [1] => lime
    [2] => blue
    [3] => yellow
)
yellow

print_r() prints arrays in a human-friendly format and is one of the most useful debugging tools in PHP.

Associative Arrays

Associative arrays use string keys, which makes them perfect for records like a user or a product. Use isset() or array_key_exists() to check if a key exists. The difference: isset() returns false when the value is null.

<?php

$user = [
    "name" => "Siti",
    "email" => "siti@example.com",
    "age" => 28,
];

echo $user["name"] . "\n";
$user["city"] = "Surabaya";   // add a new key
unset($user["age"]);          // remove a key

var_dump(isset($user["age"]), array_key_exists("email", $user));
print_r($user);

Output:

Siti
bool(false)
bool(true)
Array
(
    [name] => Siti
    [email] => siti@example.com
    [city] => Surabaya
)

Nested (multidimensional) arrays

Arrays can contain other arrays. This is exactly how a list of database rows or a decoded JSON response looks.

<?php

$students = [
    ["name" => "Budi", "score" => 78, "skills" => ["PHP", "SQL"]],
    ["name" => "Siti", "score" => 92, "skills" => ["PHP", "JS", "Go"]],
];

echo $students[1]["name"] . " knows " . count($students[1]["skills"]) . " languages\n";

foreach ($students as $student) {
    echo "{$student['name']}: " . implode(", ", $student['skills']) . "\n";
}

Output:

Siti knows 3 languages
Budi: PHP, SQL
Siti: PHP, JS, Go

Destructuring and spreading

Destructuring lets you unpack values into variables in one line. The spread operator ... merges arrays; later keys overwrite earlier ones.

<?php

[$first, $second] = ["Budi", "Siti"];
echo "$first and $second\n";

['name' => $name, 'city' => $city] = ['name' => 'Budi', 'city' => 'Jakarta'];
echo "$name lives in $city\n";

// Swap two variables
$a = 1;
$b = 2;
[$a, $b] = [$b, $a];
echo "a=$a, b=$b\n";

// Spread operator works with string keys since PHP 8.1
$defaults = ['theme' => 'light', 'lang' => 'en'];
$custom = ['lang' => 'id'];
print_r([...$defaults, ...$custom]);

Output:

Budi and Siti
Budi lives in Jakarta
a=2, b=1
Array
(
    [theme] => light
    [lang] => id
)

Searching and Slicing

Pass true as the third argument to in_array() and array_search() to use strict comparison. It avoids surprises like in_array("abc", [0]).

<?php

$fruits = ["apple", "banana", "mango"];

var_dump(in_array("mango", $fruits, true));
var_dump(array_search("banana", $fruits, true));
print_r(array_keys(["a" => 1, "b" => 2]));
print_r(array_values(["a" => 1, "b" => 2]));
print_r(array_slice([10, 20, 30, 40, 50], 1, 3));
print_r(array_merge([1, 2], [3, 4]));

Output:

bool(true)
int(1)
Array
(
    [0] => a
    [1] => b
)
Array
(
    [0] => 1
    [1] => 2
)
Array
(
    [0] => 20
    [1] => 30
    [2] => 40
)
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
)

Transforming: array_map, array_filter, array_reduce

These three functions replace most hand-written loops:

• array_map($fn, $array) transforms every element and returns a new array of the same size.
• array_filter($array, $fn) keeps only elements for which the callback returns true.
• array_reduce($array, $fn, $initial) combines all elements into a single value.

Watch the argument order: array_map takes the callback first, while array_filter and array_reduce take the array first.

<?php

$prices = [12000, 5000, 30000, 7500];

$withTax = array_map(fn($p) => $p * 1.1, $prices);
print_r($withTax);

$expensive = array_filter($prices, fn($p) => $p >= 10000);
print_r($expensive);                 // keys are preserved!
print_r(array_values($expensive));   // re-index

$total = array_reduce($prices, fn($carry, $p) => $carry + $p, 0);
echo "Total: $total\n";

Output:

Array
(
    [0] => 13200
    [1] => 5500
    [2] => 33000
    [3] => 8250
)
Array
(
    [0] => 12000
    [2] => 30000
)
Array
(
    [0] => 12000
    [1] => 30000
)
Total: 54500

Notice that array_filter keeps the original keys (0 and 2). If you need a clean list, wrap it with array_values(), otherwise json_encode will turn it into a JSON object instead of an array.

array_column

array_column extracts one field from a list of records, optionally keyed by another field. Combined with array_filter, it reads almost like a query.

<?php

$users = [
    ["id" => 1, "name" => "Budi", "active" => true],
    ["id" => 2, "name" => "Siti", "active" => false],
    ["id" => 3, "name" => "Andi", "active" => true],
];

print_r(array_column($users, "name"));
print_r(array_column($users, "name", "id"));

$activeNames = array_column(
    array_filter($users, fn($u) => $u["active"]),
    "name"
);
echo implode(", ", $activeNames) . "\n";

Output:

Array
(
    [0] => Budi
    [1] => Siti
    [2] => Andi
)
Array
(
    [1] => Budi
    [2] => Siti
    [3] => Andi
)
Budi, Andi

Sorting

PHP's sort functions modify the array in place and return true, not the sorted array. Choose the function based on whether you want to keep keys:

Function	Sorts by	Keeps keys?
sort / rsort	Value	No
asort / arsort	Value	Yes
ksort / krsort	Key	Yes
usort / uasort	Custom callback	No / Yes

<?php

$numbers = [5, 3, 9, 1];
sort($numbers);              // sorts in place, returns bool
echo implode(", ", $numbers) . "\n";
rsort($numbers);
echo implode(", ", $numbers) . "\n";

$ages = ["Budi" => 30, "Siti" => 25, "Andi" => 35];
asort($ages);                // sort by value, keep keys
print_r($ages);
ksort($ages);                // sort by key
print_r($ages);

Output:

1, 3, 5, 9
9, 5, 3, 1
Array
(
    [Siti] => 25
    [Budi] => 30
    [Andi] => 35
)
Array
(
    [Andi] => 35
    [Budi] => 30
    [Siti] => 25
)

For custom ordering, usort combined with the spaceship operator <=> is the idiomatic approach:

<?php

$products = [
    ["name" => "Laptop", "price" => 12000000],
    ["name" => "Mouse", "price" => 150000],
    ["name" => "Monitor", "price" => 2500000],
];

usort($products, fn($a, $b) => $a["price"] <=> $b["price"]);

foreach ($products as $p) {
    echo str_pad($p["name"], 10) . number_format($p["price"], 0, ',', '.') . "\n";
}

// Descending: swap $a and $b
usort($products, fn($a, $b) => $b["price"] <=> $a["price"]);
echo "Most expensive: " . $products[0]["name"] . "\n";

Output:

Mouse     150.000
Monitor   2.500.000
Laptop    12.000.000
Most expensive: Laptop

More handy functions

<?php

$tags = ["php", "web", "php", "api", "web"];

print_r(array_unique($tags));
print_r(array_count_values($tags));
print_r(array_combine(["a", "b"], [1, 2]));
print_r(array_chunk([1, 2, 3, 4, 5], 2));
echo json_encode(["name" => "Budi", "skills" => ["PHP", "SQL"]]) . "\n";

Output:

Array
(
    [0] => php
    [1] => web
    [3] => api
)
Array
(
    [php] => 2
    [web] => 2
    [api] => 1
)
Array
(
    [a] => 1
    [b] => 2
)
Array
(
    [0] => Array
        (
            [0] => 1
            [1] => 2
        )

    [1] => Array
        (
            [0] => 3
            [1] => 4
        )

    [2] => Array
        (
            [0] => 5
        )

)
{"name":"Budi","skills":["PHP","SQL"]}

Common Mistakes

• Calling array_filter without a callback on data containing 0 or "0". All falsy values are removed, which may delete valid data:

<?php

$values = [0, 1, "", "hello", null, "0", false, 42];

print_r(array_filter($values));   // no callback: removes ALL falsy values

Output:

Array
(
    [1] => 1
    [3] => hello
    [7] => 42
)

• Assigning the result of sort(). $sorted = sort($arr); gives you true, not an array.
• Reading a missing key. $user["phone"] raises a warning if the key does not exist. Use $user["phone"] ?? null.
• Using array_merge with numeric keys and expecting them to be preserved. Numeric keys are renumbered. Use the + operator if you need to keep them.
• Loose search. Always pass true as the strict flag to in_array and array_search.

Exercise

Given a list of orders, calculate the total spending per customer, sort from highest to lowest, and show only customers who spent at least Rp 100.000.

<?php

$orders = [
    ["customer" => "Budi", "total" => 150000],
    ["customer" => "Siti", "total" => 320000],
    ["customer" => "Budi", "total" => 90000],
    ["customer" => "Andi", "total" => 45000],
    ["customer" => "Siti", "total" => 80000],
];

$perCustomer = [];
foreach ($orders as $order) {
    $perCustomer[$order["customer"]] = ($perCustomer[$order["customer"]] ?? 0) + $order["total"];
}

arsort($perCustomer);
$big = array_filter($perCustomer, fn($total) => $total >= 100000);

foreach ($big as $customer => $total) {
    echo "$customer: " . number_format($total, 0, ',', '.') . "\n";
}

Output:

Siti: 400.000
Budi: 240.000

Challenge: rewrite the grouping step with array_reduce instead of foreach.

Summary

• PHP arrays can be indexed lists, associative maps, or nested structures.
• Use print_r/var_dump to inspect, isset/?? to read safely.
• array_map, array_filter, array_reduce, and array_column replace many manual loops.
• Sorting functions work in place; pick the one that keeps keys when you need them, and use usort with <=> for custom order.

In Part 6 we will put arrays to work with real user input: handling HTML forms, $_GET and $_POST, validation, and safely escaping output.