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$app->get('/markers/?', function () use ($app) {
    $controller = new UFMarkersController($app);
    return $controller->getMarkersJSON();
});

<!DOCTYPE html>
<html lang="en">
    <head>
    {% include 'components/head.html' %}
    </head>
    <body>
<?php
include('DB_INFO.php');

function getMarkersJSON(){
// Opens a connection to a MySQL server.
$connection = mysqli_connect($server, $username, $password);

if (!$connection) {
    die('Not connected : ' . mysqli_error());}
// Sets the active MySQL database.
$db_selected = mysqli_select_db($database, $connection);

if (!$db_selected) {
    die('Can't use db : ' . mysqli_error());}

// Selects all the rows in the markers table.
$query = "SELECT * FROM tester WHERE 1";
$result = mysqli_query($connection, $query);

if (!$result) {
    die('Invalid query: '. mysqli_error());
}
$markers = array();
while ($row = mysqli_fetch_assoc($result)) {
    //Assuming "lat" is column name in tester table. Please change it if required.
    $lat= $rows['lat'];
    //Assuming "lng" is column name in tester table. Please change it if required.
    $lng= $rows['lng'];
    $markers = array('lat' => $lat, 'lng' => $lng);
}
echo json_encode($markers);
}
?>
 </body>
</html>

<meta http-equiv="Content-Type" content="text/html;charset=[your-charset]" />