Creating a client (Appstudio app) communicate with MySQL database

Dear all.
The setup is the following:
I am trying to make an android app communicate with my MySql database Server (which for now is on localhost:3308).

On the client side I have created the following code:
dataString = {uname: uname.value, pass: pass.value, email:email.value};

When a button is clicked, the code gathers the necessary information and send it to register.php.

            type: "GET",
            url: "register.php",
            data: dataString, 
            cache: false,

            success: function(d){
             error: function(XMLHttpRequest, textStatus, errorThrown) { 
                        alert("Status: " + textStatus); alert("Error: " + errorThrown); 


     // Get the data from the client. 
    $servername = "localhost:3308";
    $username = "root";
    $password = "";
    $dbname = "housecontrol";

    // Create connection
    $conn = new mysqli($servername, $username, $password, $dbname);
    // Check connection
    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);

    $sql = "INSERT INTO 'users'(user_name,password,user_id,email,full_name,address,telephone,comments) VALUES ('john','1234','john73','','john smith','Johnson Str','+0177748883','blah blah')";
    if ($conn->query($sql) === TRUE) {
        echo "New record created successfully";
    } else {
        echo "Error: " . $sql . "<br>" . $conn->error;
  1. Where should the register.php reside? In the www root folder of the apache server? Somewhere else?
  2. In short, what does it take to make an app, running from an X network, communicate with a MySQL DB in a Y network?