why does PHP json_encode ignore empty arrays?

Question

I have the following javascript that sends data to a PHP function:

<script>
    var mydata = {
                   id:123,
                   name: 'mike',
                   orders: []
    };

    $.ajax({
       type: 'POST',
       url:  'test.php',
       data: {save_data:mydata},
       success: function(data) {
                   alert('php received: ' + data);
       }
    });
</script>

and my test.php file contains the following code:

<?php
     if (isset($_POST['save_data'])) {
          $json = json_encode($_POST['save_data']);
          echo $json;     // just to check what has been received
          exit();
     }
?>

What I expect to received from PHP is: {"id":"123","name":"mike","orders":"[]"}

What I got back is {"id":"123","name":"mike"}

Notice that orders array has been eliminated from the output. No place holder for it. I tried adding some dummy elements in the array, and that worked fine, and I received the array back with the elements.

I need PHP to receive the json object as is, even if it contains empty arrays.

How can I do that?

Answer 1

The JSON object is created inside PHP. Before then you just have form data.

jQuery will encode form data in a PHP-friendly style.

If you give it:

data: { foo: [1, 2, 3] }

It will convert that to:

foo[]=1&foo[]=2&foo[]=3

(although it will percent encode the [])

You get a key=value pair for each value.

If you have an empty array then you don't have any values, so you don't get any key=value pairs.

There is no way to encode "an empty array" using PHP's extensions to the form url encoding syntax.

---

You have two basic options:

1. Tell PHP about what data structure you want to create in advance and have it fill in the empty arrays.
2. Generate the JSON on the client