This is how json_encode() treats arrays...

json.php

<?php

$a = new StdClass();
$a->prop = 2;
$a->str = "hallo";
$a->arr = array(1,2,3);

echo json_encode($a); // {"prop":2,"str":"hallo","arr":[1,2,3]}
echo json_encode((array)$a); // {"prop":2,"str":"hallo","arr":[1,2,3]}
echo json_encode(array(1,2,3)); // [1,2,3]