I've got some very weird behavior in my PHP code. I don't know if this is actually a good SO question, since it almost looks like a bug in PHP. I had this problem in a project of mine and isolated the problem: // json object that will be converted into an array $json = '{"5":"88"}'; $jsonvar = (array) json_decode($json); // notice: Casting to an array // Displaying the array: var_dump($jsonvar); // Testing if the key is there var_dump(isset($jsonvar["5"])); var_dump(isset($jsonvar[5])); That code outputs the following: array(1) { ["5"]=> string(2) "88" } bool(false) bool(false) The big problem: Both of those tests should produce bool(true) - if you create the same array using regular php arrays, this is what you'll see: // Let's create a similar PHP array in a regular manner: $phparr = array("5" => "88"); // Displaying the array: var_dump($phparr); // Testing if the key is there var_dump(isset($phparr["5"])); var_dump(isset($phparr[5])); The output of that: array(1) { [5]=> string(2) "88" } bool(true) bool(true) So this doesn't really make sense. I've tested this on two different installations of PHP/apache. You can copy-paste the code to a php file yourself to test it. It must have something to do with the casting from an object to an array.