Array Functions: sort, filter, and map

A post office sorting machine takes a pile of letters and, in one go, puts them in order, throws out the ones with no address, or stamps every single one. You don't handle each letter yourself.

In the last two lessons, you wrote foreach loops to total, search, and filter arrays. That works, but the same jobs come up again and again, so PHP has built-in functions for them. They're shorter, and they say what they do.

Quick numbers

<?php
$scores = [78, 92, 85, 64, 92];

echo "Count: " . count($scores) . "\n";
echo "Total: " . array_sum($scores) . "\n";
echo "Highest: " . max($scores) . "\n";
echo "Lowest: " . min($scores) . "\n";
echo "Average: " . array_sum($scores) / count($scores) . "\n";

Count: 5
Total: 411
Highest: 92
Lowest: 64
Average: 82.2

range(1, 5) makes [1, 2, 3, 4, 5], and array_unique removes repeated values.

Sorting

sort puts an array in order, smallest first (or A to Z). rsort does the reverse:

<?php
$names = ["Carlo", "ana", "Ben"];
sort($names);
print_r($names);

$scores = [78, 92, 85];
rsort($scores);
echo implode(", ", $scores), "\n";

Array
(
    [0] => Ben
    [1] => Carlo
    [2] => ana
)
92, 85, 78

Two things to notice:

• The sort functions change the array itself. They don't return a new, sorted copy. (They return true, so $sorted = sort($names); is a common mistake that stores true.)
• "ana" came last: sorting compares character codes, and all capital letters come before all lowercase ones. For a case-insensitive sort, use sort($names, SORT_FLAG_CASE | SORT_STRING), or better, store names consistently.

Sorting associative arrays

sort throws away the keys and renumbers from 0. For associative arrays, use:

• asort / arsort: sort by value, keeping each key with its value.
• ksort / krsort: sort by key.

<?php
$votes = ["Adobo" => 12, "Sinigang" => 19, "Kare-kare" => 7];

arsort($votes);
print_r($votes);

ksort($votes);
echo implode(", ", array_keys($votes)), "\n";

Array
(
    [Sinigang] => 19
    [Adobo] => 12
    [Kare-kare] => 7
)
Adobo, Kare-kare, Sinigang

Your own order: usort

What about sorting a list of records, like students by grade? usort lets you decide the order. You give it a small function that compares two items, returning a negative number if the first should come first, a positive number if the second should, or 0 if it doesn't matter. The spaceship operator <=>, from Operators and Expressions, gives exactly that:

<?php
$students = [
    ["name" => "Maria", "grade" => 88],
    ["name" => "Ben", "grade" => 95],
    ["name" => "Carlo", "grade" => 79],
];

usort($students, fn($a, $b) => $b["grade"] <=> $a["grade"]);

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

Ben: 95
Maria: 88
Carlo: 79

fn($a, $b) => ... is an arrow function: a small function with no name, written right where it's needed. It takes the parameters in parentheses, and returns whatever comes after the =>. Here, $b <=> $a (b first) sorts from highest to lowest. Swap them to $a["grade"] <=> $b["grade"] for lowest first.

Keep some: array_filter

array_filter keeps only the items for which your function returns true:

<?php
$prices = [45, 120, 30, 250, 80];
$cheap = array_filter($prices, fn($p) => $p < 100);
print_r($cheap);

Array
(
    [0] => 45
    [2] => 30
    [4] => 80
)

Notice that the kept items keep their original keys: 0, 2, and 4. That's fine for foreach, but if you need them numbered 0, 1, 2 again, wrap the result in array_values().

Change every one: array_map

array_map runs your function on every item, and gives back a new array of the results:

<?php
$prices = [100, 250, 80];
$withTax = array_map(fn($p) => $p * 1.12, $prices);
$labels = array_map(fn($p) => "PHP " . number_format($p, 2), $withTax);
echo implode(" | ", $labels), "\n";

PHP 112.00 | PHP 280.00 | PHP 89.60

Careful: the order of the arguments differs. array_map takes the function first, and array_filter takes the array first. Everyone mixes these up; check the order when you get a strange error.

One column: array_column

For a list of records, array_column pulls out one field from each:

<?php
$students = [
    ["name" => "Maria", "grade" => 88],
    ["name" => "Ben", "grade" => 95],
];
echo implode(", ", array_column($students, "name")), "\n";
echo "Class average: " . array_sum(array_column($students, "grade")) / count($students) . "\n";

Maria, Ben
Class average: 91.5

Chaining them together

These functions combine well. Here's "the names of everyone who passed, in alphabetical order", with no loop:

<?php
$students = [
    ["name" => "Maria", "grade" => 88],
    ["name" => "Ben", "grade" => 71],
    ["name" => "Carlo", "grade" => 79],
    ["name" => "Ana", "grade" => 93],
];

$passed = array_filter($students, fn($s) => $s["grade"] >= 75);
$names = array_column($passed, "name");
sort($names);
echo implode(", ", $names), "\n";

Ana, Carlo, Maria

When a chain gets hard to read, break it into steps with well-named variables, as here. A plain foreach is never wrong, either. Use whichever makes your code clearest.

Try it

A running club looks at race times, in minutes. 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

Fastest: 27 min
Average: 30.5 min
Podium: Marco, Paolo, Lia
Under 30 min: 2
MARCO (27) > PAOLO (29) > LIA (31) > JOY (35)

usort with $a <=> $b puts the smallest time first. array_slice($runners, 0, 3) takes the first 3 records, and array_column pulls their names. Two runners finished under 30, and array_map runs on the already-sorted list, so the badges come out in finishing order.

Try it yourself

1. Add a runner, ["name" => "Bea", "time" => 26]. Who's on the podium now?
2. Use array_filter to find runners slower than the average.
3. Use array_map to make a list of just the first letter of each name, and print it with implode.

Check your understanding

What does $result = sort($names); store in $result?

The sorted array

true

The first name

null

Which function keeps only the items that pass a test?

array_map

array_column

array_filter

usort

Which usort comparison sorts records from highest score to lowest?

fn($a, $b) => $b["score"] <=> $a["score"]

fn($a, $b) => $a["score"] <=> $b["score"]

fn($a, $b) => $a["score"] > $b["score"]

fn($a) => $a["score"]

Up next

So far, your scripts have run in the terminal. Time to take PHP back to the web, and let visitors send you data, in Handling Forms: GET and POST.