Return a tuple from MySQL database with statements
The whole processServerResponse() method looks rather alien to me. I never seen anything like that. A server response shouldn't be so much tightly coupled with such a specific stuff like a statement class for one of PHP extensions. A server  response should be from a server, not a statement.

Besides, you are mixing two response codes here, "error occurred" and "zero results found". It should be different codes.

Usually, errors are checked by a distinct error handler, while number of results found can be checked within the same routine that returns the data.

So I would make it this way

public function getManufacturer($id) {
    $stmt = $this-&gt;dbConnection-&gt;prepare("SELECT ID, URL FROM ERP.MANUFACTURER WHERE ID = ?");
    $stmt-&gt;bind_param("i", $id);
    $stmt-&gt;execute();
    $stmt-&gt;store_result();
    $stmt-&gt;bind_result($id, $url);
    $found = $stmt-&gt;fetch();
    return [
                ACTION_RESULT =&gt; (int)$found,
                "data" =&gt;
                [
                    "id" =&gt; $id,
                    "url" =&gt; $url
                ]
            ];
}


while errors should be checked with a site-wide handler like this (taken from my article on writing Db wrappers)

set_error_handler("myErrorHandler");
function myErrorHandler($errno, $errstr, $errfile, $errline)
{
    error_log("[$errno] $errstr in $errfile:$errline");
    header('HTTP/1.1 500 Internal Server Error', TRUE, 500);
    header('Content-Type: application/json');
    echo json_encode(["error" =&gt; 'Server error']);
    exit;
}


So this way you will get different responses for the different scenarios. 

On a side note, an obligatory suggestion: consider to use PDO instead of mysqli. This alone will do a huge refactoring for your code:

public function getManufacturer($id) {
    $stmt = $this-&gt;pdo-&gt;prepare("SELECT ID id, URL url FROM ERP.MANUFACTURER WHERE ID = ?");
    $stmt-&gt;execute([$id]);
    $data = $stmt-&gt;fetch();
    return [
                ACTION_RESULT =&gt; (int)(bool)$data,
                "data" =&gt; $data;
            ];
}