php - JSON_ENCODE of multidimensional array giving different results

Question

When doing a json_encode a multidimensional array in PHP, I'm noticing a different output simply by naming one of the arrays, as opposed to not naming them. For Example:

$arrytest = array(array('a'=>1, 'b'=>2),array('c'=>3),array('d'=>4));
json_encode($arrytest)

gives a single array of multiple json objects

[{"a":1,"b":2},{"c":3},{"d":4}];

---

whereas simply assigning a name to the middle array

$arrytest = array(array('a'=>1, 'b'=>2),"secondarray"=>array('c'=>3),array('d'=>4));
json_encode($arrytest)

creates a single json object with multiple json objects inside

{"0":{"a":1,"b":2},"secondarray":{"c":3},"1":{"d":4}};

why would the 1st option not return the same reasults as the 2nd execpt with "1" in place of "secondarray"

Answer 1

In JSON, arrays [] only every have numeric keys, whereas objects {} have string properties. The inclusion of a array key in your second example forces the entire outer structure to be an object by necessity. The inner objects of both examples are made as objects because of the inclusion of string keys a,b,c,d.

If you were to use the JSON_FORCE_OBJECT option on the first example, you should get back a similar structure to the second, with the outer structure an object rather than an array. Without specifying that you want it as an object, the absence of string keys in the outer array causes PHP to assume it is to be encoded as the equivalent array structure in JSON.

$arrytest = array(array('a'=>1, 'b'=>2),array('c'=>3),array('d'=>4));

// Force the outer structure into an object rather than array
echo json_encode($arrytest , JSON_FORCE_OBJECT);

// {"0":{"a":1,"b":2},"1":{"c":3},"2":{"d":4}}