MySQL Returning null when value = 0?

This is a strange issue. When executing queries it will return null where a value of 0 (type int) is found.

Here is my mysql function:

public function query( $query_string, $params = null, $param_types = null) {

            //prepare the statement
            $stmt = $this->db->prepare($query_string);

            //check if the sql query is wrong.
            if($stmt === false) {
                echo "Wrong SQL: " . $query_string;
                if(DEBUG)
                    echo "<br />Error: " . $this->db->errno . " " . $this->db->error;

                return;
            }

            if($params != null && $param_types != null) {

                //build the array that needs to pass the args to bind_param.
                $a_params = array();
                $a_params[] = &$param_types;

                for($i=0; $i<count($params); $i++)
                    $a_params[] = &$params[$i];

                // $stmt->bind_param('s', $param); equivalent
                call_user_func_array(array($stmt, 'bind_param'), $a_params);
            }

            //run the query
            $stmt->execute();

            //bind the result
            $data = $stmt->result_metadata();
            $fields = $out = $results = array();
            $count = 0;
            $k = 0;

            if( $data != null ) {
                while($field = $data->fetch_field())
                    $fields[$count++] = &$out[$field->name];

                call_user_func_array(array($stmt, 'bind_result'), $fields);

                // fetch the result
                while ($stmt->fetch())
                    $results[$k++] = array_flip(array_flip($out));
            }

            $stmt->close();

            return $results;
        }

    }

MySQL Query:

public function getMenuItems( $id ) {
            $items = $this->db->query("SELECT id, menu, parent_id, name, url, external, position FROM menu_meta WHERE menu = ? ORDER BY position ASC", array($id), "i");
            return $items;
        }

My Database has the following items in the table: https://gyazo.com/d9a0a9472b0b51d43aafeafdd28a75c8

As you can see, I need values with 0 to be displayed. They are not null!

Using json_encode this is the output:

[{"menu":1,"position":0,"name":"Home","url":"..\/"},{"id":5,"menu":1,"parent_id":2,"name":"Add New","url":"#","position":0},{"id":2,"position":1,"external":0,"name":"Customers","url":"..\/?page=customers"},{"id":3,"menu":1,"external":0,"name":"Subscriptions","url":"..\/?page=subscriptions","position":2},{"id":4,"menu":1,"external":0,"name":"Billing","url":"..\/?page=billing","position":3}]

Is this a configuration issue with the server? Or perhaps something with my version of PHP?

EDIT: There is no value of "null" but rather the key + value are missing altogether. The first entry in the JSON is missing key "id" because value is 0. The others are missing "parent_id" because value is 0.