Arrays — One Type, Two Roles

Most languages ship a list type and a dictionary type. PHP ships one: array. It is an ordered map — a sequence of key/value pairs that remembers the order you inserted them. A "list" is simply an array whose keys happen to be 0, 1, 2, ....

<?php
$weights = [1.75, 12.5, 0.4];                 // keys 0, 1, 2
$depot   = ["Lisbon" => 12, "Porto" => 5];    // string keys

Both are the same type. array() is the older spelling of []; you will meet it in old code and the two are identical.

Building and reading

<?php
$weights = [1.75, 12.5, 0.4];
$weights[] = 3.0;                 // append using the next integer key
echo count($weights), "\n";
echo $weights[0], "\n";

$depot = ["Lisbon" => 12, "Porto" => 5];
$depot["Faro"] = 1;               // add or overwrite by key
print_r($depot);

//> 4
//> 1.75
//> Array
//> (
//>     [Lisbon] => 12
//>     [Porto] => 5
//>     [Faro] => 1
//> )

The bracket-with-nothing-inside form is the standard append. print_r() is the readable dump; var_dump() is the one that also shows types.

What happens when the key is missing?

PHP complains — a notice on 7.4, a warning on PHP 8 — and hands you null. Ask before you reach:

unset leaves a hole

Removing an element does not renumber the ones after it. This is the array behaviour that surprises people most.

<?php
$ids = [101, 102, 103];
unset($ids[1]);
print_r($ids);
print_r(array_values($ids));

//> Array
//> (
//>     [0] => 101
//>     [2] => 103
//> )
//> Array
//> (
//>     [0] => 101
//>     [1] => 103
//> )

array_values() throws the keys away and renumbers from zero. One more detail worth carrying: the next auto-index is one past the highest integer key that has ever existed, so appending to the array above produces key 3, not 2.

Keys get cast

Only int and string can be keys. Anything else is converted first, which means two keys you thought were different can silently collide.

Iterating

<?php
$depot = ["Lisbon" => 12, "Porto" => 5, "Faro" => 1];
foreach ($depot as $city => $count) {
    echo "$city: $count\n";
}

//> Lisbon: 12
//> Porto: 5
//> Faro: 1

Insertion order, always — for numeric and string keys alike. Nothing is sorted unless you sort it.

The toolbox

<?php
$nums = [3, 8, 2, 9, 1];
echo max($nums), " ", min($nums), " ", array_sum($nums), "\n";
echo max([-3, -1, -7]), "\n";

//> 9 1 23
//> -1

Your exercise

Largest Number reads one line of space-separated integers and prints the biggest one.

The starter's line array_map('intval', explode(' ', $line)) splits the text on spaces and converts each piece to an int. (array_map gets a lesson of its own next; here it just hands you $nums ready to use.) max() accepts a whole array, so the answer is a single line.

The mistake the grader catches is rolling your own loop seeded with $max = 0. The hidden test feeds -3 -1 -7 and expects -1, and a zero seed prints 0 because zero beats every negative value in the list. If you do write the loop, seed it with the first element instead. Print just the number, then a newline.