Accessing a variable from inside a jQuery drop dow

I have a jQuery dropdown list that has two dropdowns. The first one is a list of cars and the second one shows the car's models. This is all working fine. Now I would like to use the selection selected by the user to search a database (MySQL) and find a specific element within the database. This just gives me the else within my code and once it gave me a undefined variable error message.

How can I code this so that I can access the variables and display a search result? Here is my code:

This is just an example of the jQuery I'm using - I'm actually using cars but this shows cities:

<select class="select" id="province" onchange="filterCity();">
  <option value="1">RM</option>
  <option value="2">FI</option>
</select>

 <select class="select" id="city" disabled>
  <option data-province="RM" value="1">ROMA</option>
  <option data-province="RM" value="2">ANGUILLARA SABAZIA</option>
  <option data-province="FI" value="3">FIRENZE</option>
  <option data-province="FI" value="4">PONTASSIEVE</option>
</select>

<span id="option-container" style="visibility: hidden; position:absolute;"></span>

<script>

    function filterCity(){
      var province = $("#province").find('option:selected').text(); // stores province
      $("#option-container").children().appendTo("#city"); // moves <option> contained in #option-container back to their <select>
      var toMove = $("#city").children("[data-province!='"+province+"']"); // selects city elements to move out
      toMove.appendTo("#option-container"); // moves city elements in #option-container
      $("#city").removeAttr("disabled"); // enables select
 };
</script>

And here is the code where I'm trying to access the searched for variable (this code is echoing wrong (the else)): I used the name="car" in the second select:

if (isset($_POST['car'])) {
        $_SESSION['car'] = $_POST['car'];
    $car = $_SESSION['car'];
}
function find_tire(){
    if (isset($_POST['car'])) {
        $_SESSION['car'] = $_POST['car'];
    $car = $_SESSION['car'];
}

    global $db;

    if (isset($car)) {
    $query = $db->prepare("SELECT idtires FROM vehicle WHERE idcarmodel = $car");
 $query->execute();
    $tire = $query->fetchAll();
    var_dump($query);}
if (isset($tire)) {
            echo "<ul>";
        foreach ($tire as $name) {
                echo "<li id='tiresearch'>";
                echo "Tire Size is Available: " . $name    ['idtires'];
                echo "</li>";
            }echo "</ul>";
    } else {

            echo "Wrong";
        }
  }

回答1:

you can use ajax for this to send your variable to another page and get response from there .something like this

var id = $('selector').val();
    var AjaxURL = 'http://example.com';
    var data = id;
    $.ajax({
        type: "POST",
        url: AjaxURL,
        data: {data: data},
        success: function(result) {
            window.console.log('Successful');
        }
    });

回答2:

I had to put all the select inside form tags, use a different page for the search and add the function to the bottom of the search page instead of accessing it from a different page.

Thank you to anyone who posted answers and tried to help me with this. It's working (with a few bugs) but I'm so happy it's finally working.