mysql query doesn't work together with cross domain in ajax

my problem is the following... I made some php code that reads records from a mysql database:

$host = $_SESSION['host'];
$username = $_SESSION['username'];
$password = $_SESSION['password'];
$db = $_SESSION['db'];

$con = mysqli_connect($host,$username,$password,$db);

$sql = "SELECT * FROM `table`";
$result = $con->query($sql);
if ($result->num_rows > 0) {
    while($row = $result->fetch_assoc()) {
         ...
    }
}

my html:

<button onclick="list">submit</button>
<div id="output"></div>

<script>
var thisHost = "host.php";
function list() {
    $.ajax({
        type: 'POST',
        url: thisHost,
        data: "list=true",
        success: function(data) 
        {
            $("#output").html(data);
        },
        error: function (responseData, textStatus, errorThrown) 
        {
            console.warn(responseData, textStatus, errorThrown);
        }
    });
}
</script>

as far as that everything is working fine. However now i want to call the php code from another host. Therefor the html file is on the other host (localhost:81) and the php file stays on the old host (localhost:80).

now I noticed that i need a something to cross the domain because ajax doesn't seem to work with other hosts. So all i did was adding these lines to the php code right at the beginning:

header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Methods: POST, GET, OPTIONS');
header('Access-Control-Max-Age: 1000');
header('Access-Control-Allow-Headers: Content-Type');

and change the value of the thisHost var in javascript to

"http://localhost:80/host.php"

after that the ajax part works again and the php response. However every time i want to make a query by doing something like this:

$sql = "SELECT * FROM `table`";

i will get this error:

Fatal error: Uncaught Error: Call to a member function query() on null in C:pathtohost.php:71 Stack trace: #0 {main} thrown in C:pathtohost.php on line 71

i'm pretty sure that the query is correct. So what do i need to change?