But that adds a "0" then the KVP.

array_push() could be used if the array was a numeric array (i.e. the keys were integers - e.g. 0 => 'cat', 1 => 'dog'. But in this case, the array is an associative array. One way to set the desired key, Sandbox, is to set the array key by specifying it in bracket notation:

if ($isSandbox) {
    $sandboxkey = array( "Sandboxed" => true );
    $package['Sandbox'] = $sandboxkey;
}

This is necessary because the other keys of the array are not

See it demonstrated in this playground example.

Otherwise array_merge() could be used:

if ($isSandbox) {
    $sandboxArray = array(
    'Sandbox' => array( "Sandboxed" => true )
    );
    $package = array_merge($package, $sandboxArray);
}

See a demonstration in this playground example.

Refer to this answer for more information about numeric vs associative arrays. It is somewhat surprising that the difference isn't explicitly spelled out on the PHP Array data type page.

Additionally, I want the Errors to be listed in an array...

For this, nest $error (an associative-array, which will get converted to a JSON object) in another array.

$package = array(
    "Data" => $message,
    "Sandbox" => $sandboxkey,
    "Errors" => array($error)
);

You have to make few changes in your code, which will help you achieve your expected output.

1. Use array_merge instead of array_push

2. Set Errors key like this "Errors" => array($error) instead of "Errors" => $error

Try this code snippet here

<?php

ini_set('display_errors', 1);
$isSandbox=true;
$message = "Message was sent on 12-31-1999 12:00:00 am";
$errorMsg = "Message bounced back";
$error = array("Code" => 5, "Message" => $errorMsg);
$package = array(
    "Data" => $message,
    "Errors" => array($error)
);
if ($isSandbox)
{
    $sandboxkey = array("Sandboxed" => true);
    $package=array_merge($package, $sandboxkey);
}
echo json_encode($package,JSON_PRETTY_PRINT);

Output:

{
    "Data": "Message was sent on 12-31-1999 12:00:00 am",
    "Errors": [
        {
            "Code": 5,
            "Message": "Message bounced back"
        }
    ],
    "Sandboxed": true
}

How to handle json:

• [] are arrays in json
• {} are objects in json

So to create something like {'data':[{'depperdata':2}]} do

 $a = new stdClass();
 $a->data = array();
 $b = new stdClass();
 $b->deeperdata=2;
 $a->data[0]=$b;
 print_r(json_encode($a,JSON_PRETTY_PRINT));

or

 $a = array('data'=>array(array('depperdata'=>2)));
 print_r(json_encode($a,JSON_PRETTY_PRINT));

What does json_decode

• if you have an assoc-array it becomes an json_object
• if you have an indexed array it becomes an json_array
• if you have an object it becomes an json_object
• if you have mixed assoc/index array you are getting broken json like your example. property name 0! (prevent that!)

Have a nice day...