PHP 8.1: SplFixedArray implements JsonSerializable, and json-encodes as an array


SplFixedArray provides the functionality of a fixed array, akin to other programming languages. The difference between a standard PHP array and an SplFixedArray is that that SplFixedArray only allows a fixed number of elements, and the keys will be always integers in the range.

Although a SplFixedArray class instances implement ArrayAccess interface and behave like an array in most programming languages, the json_encode function encoded SplFixedArray instance as objects, as opposed to arrays.

From PHP 8.1 and later, SplFixedArray implements JsonSerializable interface, and encode SplFixedArray instances as arrays.

$array = new SplFixedArray(3);
$array[] = 'Apple';
$array[] = 'Banana';
$array[] = 'Mango';

echo json_encode($array);

The output of the json_encode call will be different in PHP 8.1 and later:

PHP < 8.1


PHP >= 8.1


SplFixedArray class also implements JsonSerializable interface since PHP 8.1:

- class SplFixedArray implements Iterator, ArrayAccess, Countable {
+ class SplFixedArray implements Iterator, ArrayAccess, Countable, JsonSerializable {

Backwards Compatibility Impact

  • Since PHP 8.1, applications that relied to on the json_encode output of SplFixedArray objects to be an object notation will receive an array notation instead.

  • An SplFixedArray object now evaluates to true for instanceof JsonSerializable.

  • The output of serialize operations are unaffected.