PHP Arrays¶

PHP arrays are ordered maps - they serve as arrays, lists, hash tables, dictionaries, stacks, and queues. Three types: indexed (numeric keys), associative (string keys), and multidimensional (nested). Rich standard library with 70+ functions. Short syntax [] replaced array() in modern PHP.

Key Facts¶

All PHP arrays are ordered hash maps internally (preserve insertion order)
Short syntax: $a = [1, 2, 3] or $a = ['key' => 'value']
Array unpacking: [...$a, ...$b] merges arrays (PHP 7.4+)
array_map, array_filter, array_reduce for functional operations
list() or [$a, $b] = $arr for destructuring assignment
Arrays are passed by value (copy-on-write); use &$arr for reference

Patterns¶

Creation and Access¶

$fruits = ['apple', 'banana', 'cherry'];
$user = ['name' => 'Alice', 'age' => 30];
$matrix = [[1, 2], [3, 4]];

$fruits[] = 'date';  // append
echo $user['name'];  // Alice

Key Functions¶

in_array($needle, $haystack, strict: true);  // value search
array_key_exists('name', $user);             // key check
array_search('banana', $fruits);             // returns key

// Functional
array_map(fn($x) => $x * 2, [1, 2, 3]);           // [2, 4, 6]
array_filter([1, 0, 2, ''], fn($x) => $x > 0);    // [1, 2]
array_reduce([1, 2, 3], fn($c, $x) => $c + $x, 0); // 6

// Merge
array_merge($a, $b);              // reindex numeric keys
$a + $b;                          // union (first wins for duplicates)
array_column($records, 'name');   // extract column from 2D

// Destructuring
[$first, $second] = [10, 20];
['name' => $name, 'age' => $age] = $user;

Sorting¶

sort($arr);      // by value ascending, reindex
asort($arr);     // by value, preserve keys
ksort($arr);     // by key
usort($arr, fn($a, $b) => $a <=> $b);  // custom

Gotchas¶

array_merge reindexes numeric keys; + preserves them (first value wins for duplicates)
in_array uses loose comparison by default - pass true as third arg
array_filter without callback removes all falsy values including 0 and "0"
foreach ($arr as &$value) modifies in place - unset($value) after or last element gets overwritten