json_encode encodes a sparse array as a JSON object

Question

I'm experiencing odd behavior with json_encode after removing a numeric array key with unset:

$a = array(
    new stdclass,
    new stdclass,
    new stdclass
);
$a[0]->abc = '123';
$a[1]->jkl = '234';
$a[2]->nmo = '567';

printf("%s\n", json_encode($a));
unset($a[1]);
printf("%s\n", json_encode($a));

Program Output

[{"abc":"123"},{"jkl":"234"},{"nmo":"567"}]
{"0":{"abc":"123"},"2":{"nmo":"567"}}

The first time $a is converted to JSON, it's encoded as an array. The second time, it's encoded as an object. Why is this happening, and how can I prevent it?

Answer 1

The reason for that is that your array has a hole in it: it has the indices 0 and 2, but misses 1. JSON can't encode arrays with holes because the array syntax has no support for indices.

You can encode array_values($a) instead, which will return a reindexed array.

Answer 2

In addition to the array_values technique it is possible to use array_splice and remove an element and re-index in one step:

unset($a[1]);

Instead:

array_splice($a, 1, 1);

Answer 3

Try to use JSON_FORCE_OBJECT option for json_encode, like: json_encode($a, JSON_FORCE_OBJECT) so you will always have the same result.