Array Behaviour You Should Know

A few rules about how PHP arrays behave catch many beginners. Learn them once.

Arrays are copied on assignment

Unlike objects, arrays are values. Assigning an array to another variable gives you an independent copy.

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

$b[] = 4;

echo count($a) . " vs " . count($b);

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3 vs 4

The same is true when you pass an array to a function. The function changes its own copy (unless you use &).

Keys are converted

PHP changes some keys to integers.

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<?php
$data = [];
$data["5"] = "text five";   // becomes the integer 5
$data[true] = "true";       // becomes 1
$data[null] = "null";       // becomes ""

var_dump(array_keys($data));

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array(3) {
  [0]=>
  int(5)
  [1]=>
  int(1)
  [2]=>
  string(0) ""
}

A key like "05" stays a string, because it is not a plain integer.

Duplicate keys overwrite

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<?php
$a = ["x" => 1, "x" => 2];

echo $a["x"];

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The next free index

When you add with [], PHP uses the highest integer key plus one.

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<?php
$list = [5 => "a"];
$list[] = "b";

print_r($list);

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Array
(
    [5] => a
    [6] => b
)

Order is kept

PHP arrays remember the order you added items in. That is why foreach gives items back in that order. Sorting functions change it.

Nested arrays are copied too

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<?php
$config = ["db" => ["host" => "localhost"]];
$copy = $config;

$copy["db"]["host"] = "remote";

echo $config["db"]["host"];

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localhost

Objects inside arrays are shared

Arrays of objects copy the array, but each object is still the same one.

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<?php
$user = new stdClass();
$user->name = "Ana";

$list = [$user];
$copy = $list;

$copy[0]->name = "Changed";

echo $list[0]->name;

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Changed

Reading a missing key

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<?php
$a = ["x" => 1];

echo $a["y"] ?? "missing";

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missing

Without ??, PHP 8 raises a warning: Undefined array key "y". Do not hide such warnings. They point to real bugs.

Arrays can be compared and counted

You saw == and === in Array Operators.