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Arrays

In PHP, an array is an ordered map that associates keys to values. A key can be an integer or a string. These two statements do the same assignment:
   $arr = array(“a”=>”Hello”, 5 => “World”);
   $arr = [“a”=>”Hello”, 5 => “World”]; // PHP 5.4+

If a key is a float, a string containing an integer or a boolean, it will be cast to an integer. If the key is not specified, it will be regarded as the largest integer index plus 1. Eg.:

   $arr = [“orange”,                         // key 0
              ”apple”,                            // key 1
              5.7=> “pineapple”,            // key 5
              “special” => “cucumber”,  // key “special”
               “watermelon”];                // key 6
   $arr[]=”pear”;                             // key 7

To access $arr, use the square brackets [], eg.: $arr[1], $arr[“special”].

Assign an array to a key in an array to create a multidimensional array.
   $arr = [“ma”=>[“foo”=>”bar”]];

   $arr[1][2] = “live”;
To access the value “foo”, use $arr[“ma”][“foo”].

Applying the + operator to two arrays returns an array which is a union of the two arrays. If the arrays contain the same keys, the key-value pairs of the left operand will be preserved.

<!DOCTYPE html>
<html>
<head></head>
<body><?php

$a=[4,5,6];
$b=[1,2,3,7,8];
var_dump ($a + $b);

?></body>
</html>

array(5) { [0]=> int(4) [1]=> int(5) [2]=> int(6) [3]=> int(7) [4]=> int(8) }
Arrays may be compared with comparison operators. 

<!DOCTYPE html>
<html>
<head></head>
<body><?php

$a=["hello","hi"];
$b=[1=>"hi",0=>"hello"];
var_dump ($a==$b);   // true; same key-value pairs
var_dump ($a===$b); // false; different order
var_dump ($a!==$b);  // true; different order
var_dump ($a<>$b);   // false; same key-value pairs
var_dump ($a!=$b);    // false; same key-value pairs

?></body>
</html>

bool(true) bool(false) bool(true) bool(false) bool(false)
The values of an array can be assigned to a list() of variables directly: 

<!DOCTYPE html>
<html>
<head></head>
<body><?php

$arr = ['apple',2=>'orange','mango'];
list($a0,,,$a3)=$arr;   // index 1 and index 2 skipped
echo ($a0." ".$a3."<br />");

list($a,list($b,$c)) = ['fruits',['vege','meat']];  // nested
echo ($a." ".$b." ".$c);

?></body>
</html>

apple mango fruits vege meat
Since PHP 7.4, the spread operator ... can be used inside an array literal to expand all elements of one array into another. Since PHP 8.1, this also works for arrays with string keys – and unlike the + union operator above, when a later spread provides the same string key as an earlier one, the later value wins (matching how array_merge() behaves):
Spreading numerically-keyed arrays re-indexes them from 0:
<?php
$a = [1, 2, 3];
$b = [0, ...$a, 4, 5];
print_r($b);
?>

Array ( [0] => 0 [1] => 1 [2] => 2 [3] => 3 [4] => 4 [5] => 5 )
Spreading string-keyed arrays (PHP 8.1+): the later "b" overwrites the earlier one:
<?php
$assoc1 = ["a" => 1, "b" => 2];
$assoc2 = ["b" => 3, "c" => 4];
$merged = [...$assoc1, ...$assoc2];
print_r($merged);
?>

Array ( [a] => 1 [b] => 3 [c] => 4 )
Since PHP 7.1, list() (and its short [] syntax) can destructure an associative array by naming which key goes into which variable, using the same key => $variable syntax as an array literal. This also means the target variables no longer need to follow the original array's order: 

<?php
$data = ['id' => 42, 'name' => 'Alice', 'role' => 'admin'];

['id' => $id, 'name' => $name] = $data;
echo "$id - $name\n";

list('name' => $n, 'role' => $r) = $data;
echo "$n - $r\n";
?>

42 - Alice Alice - admin
Since PHP 7.3, elements of a list() assignment can be prefixed with & to assign by reference instead of by value. Each such variable then points to the same value as the corresponding array element, so changing the variable changes the original array too: 

<?php
$arr = [1, 2, 3];

[&$a, &$b, $c] = $arr;
$a = 10;
$b = 20;

print_r($arr);
?>

Array ( [0] => 10 [1] => 20 [2] => 3 )