Understanding PHP json_encode Output: When Arrays Become Objects

When converting PHP arrays to JSON with json_encode, the resulting JSON may be an object rather than an array if the array indices are not a consecutive sequence starting from zero. This guide records typical problems encountered in daily development and offers concrete fixes.

Problem: Array to JSON sometimes yields an object, sometimes an array.

Case 1:

$arr = [1, 2, 3]; // zero‑based indices
echo json_encode($arr); // [1,2,3]

$arr = [1=>1, 2=>2, 3=>3]; // non‑zero‑based indices
echo json_encode($arr); // {"1":1,"2":2,"3":3}

If an array output is required, re‑index the array:

$arr = [1=>1, 2=>2, 3=>3];
echo json_encode(array_values($arr)); // [1,2,3]

Case 2:

$arr = [];
$arr[0] = 'xxx';
$arr[1] = 'yyy';
echo json_encode($arr); // ["xxx","yyy"]

$arr = [];
$arr[1] = 'yyy';
$arr[0] = 'xxx';
echo json_encode($arr); // {"1":"yyy","0":"xxx"}

Because the insertion order changes, the JSON representation differs.

Friend's attempted fix:

$arr = [];
$arr[1] = 'yyy';
$arr[0] = 'xxx';
$arr = array_values($arr);
echo json_encode($arr); // ["yyy","xxx"]

This produces the wrong order because array_values re‑indexes based on the original order.

Correct solution:

$arr = [];
$arr[1] = 'yyy';
$arr[0] = 'xxx';
ksort($arr);
echo json_encode($arr); // ["xxx","yyy"]

Case 3:

$arr = [];
$arr[0] = 'yyy';
$arr[2] = 'xxx';
print_r($arr);
echo json_encode($arr);
/*
Array
(
    [0] => yyy
    [2] => xxx
)
{"0":"yyy","2":"xxx"}
*/

Case 4:

$arr = [];
echo json_encode($arr); // []

Summary:

When array indices are not a continuous sequence starting from 0, json_encode outputs a JSON object.

To obtain a JSON array, ensure indices are consecutive from 0 without gaps, or re‑index the array using array_values or sort the keys with ksort before encoding.