包含后PHP中的未定义变量

I'm new to stackoverflow (as a member at least) and I have a question. I'm also new to PHP by the way.

Thing is: I want to dynamically fill a second dropdown list with entries based on a first dropdown list (I want to show cities based on a selected province). I managed to get the selected province to an external PHP file with an AJAX call in Javascript. But when I include the external PHP file in my original PHP file, the variable of the external file is undefined. The AJAX call is fired with an onchange event on the first dropdown menu.

And maybe you can also help with how I use that variable to get the right content in the second dropdown. I've used a multidimensional Array for that.

HTML:

echo('<select name="provincie" id="provincie"       onchange="ProvinciePHP()">');


    foreach ($provincie as $provincies){
        echo ("<option> $provincies </option>");
    }
    echo ('</select>');


echo('<select name="stad" id="stad"> </select>')

PHP ARRAY:

$provincie = array(
    'Selecteer een provincie',
    'Drenthe',
    'Flevoland',
    'Friesland',
    'Gelderland',
    'Groningen',
    'Limburg',
    'Noord-Brabant',
    'Noord-Holland',
    'Overijssel',
    'Utrecht',
    'Zeeland',
    'Zuid-Holland',
);
$stad = array(
    'Drenthe'       => array("Assen", "Emmen", "Hoogeveen", "Meppel"),
    'Flevoland'     => array("Almere", "Biddinghuizen", "Dronten", "Lelystad"),
    'Friesland'     => array("Heerenveen", "Joure", "Leeuwarden", "Sneek"),
    'Gelderland'    => array("Apeldoorn", "Arnhem", "Nijmegen", "Zutphen"),
    'Groningen'     => array("Delfzijl", "Groningen", "Stadskanaal", "Veendam"),
    'Limburg'       => array("Maastricht", "Roermond", "Sittard", "Venlo"),
    'Noord-Brabant' => array("Breda", "Den Bosch", "Eindhoven", "Tilburg"),
    'Noord-Holland' => array("Alkmaar", "Amsterdam", "Haarlem", "Hilversum"),
    'Overijssel'    => array("Deventer", "Enschede", "Hengelo", "Zwolle"),
    'Utrecht'       => array("Amersfoort", "Breukelen", "Utrecht", "Zeist" ),
    'Zeeland'       => array("Goes", "Middelburg", "Terneuzen", "Vlissingen"),
    'Zuid-Holland'  => array("Alphen a/d Rijn", "Den-Haag", "Rotterdam", "Schiedam"),
);

function getProvincie(){
var provincie = $('#provincie option:selected').val();

$.ajax({
    type: "POST",
    url: "getprovincie.php",
    data: {data: provincie},
    success: function(data) {
        alert(data);
        $('#stad').css('display','inline');
    },
    error: function(data) {

    }
});

}

//PHP page to where the AJAX call points:
<?php
$resultaat = $_POST['data'];

echo($resultaat);