Why Does `json_encode` Convert Sparse PHP Arrays to JSON Objects?

Sparse Arrays Encoded as JSON Objects in json_encode

When an array contains a missing index, it's known as a sparse array. json_encode, by default, handles sparse arrays as JSON objects instead of arrays. This can lead to unexpected behavior in various scenarios.

Cause of Array-to-Object Conversion

JSON syntax doesn't support arrays with missing indices. When json_encode encounters a sparse array, it assumes it's an object and converts it accordingly.

Example

Consider the following code:

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

// First JSON encoding
echo json_encode($a) . "\n";

// Unset an element
unset($a[1]);

// Second JSON encoding
echo json_encode($a) . "\n";

Output

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

In the first encoding, the array is converted to a JSON array as expected. However, after unsetting index 1, the second encoding results in a JSON object due to the sparse array issue.

Prevention

To prevent this behavior and ensure consistent array encoding, you can use array_values() to reindex the array before passing it to json_encode. This will remove any holes in the array and ensure it's treated as an array by json_encode.

Modified Code

echo json_encode(array_values($a)) . "\n";

This modified code will always encode $a as a JSON array, regardless of whether it has missing indices or not.

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