select2 kullanabilirsiniz.

örnek:
<html>
<head>
    <link href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/css/select2.min.css" rel="stylesheet" />
</head>
<body>
    <?php
    if(isset($_POST['dropdown-submit'])){
        echo 'seçilen değer: '.$_POST['dropdown'];
    }
    ?>
    <form method="post" action="#">
        <select class="select2" tabindex="-1" aria-hidden="true" name="dropdown">
            <option>Alabama</option>
            <option selected>Alaska</option>
            <option>California</option>
            <option>Delaware</option>
            <option>Tennessee</option>
            <option>Texas</option>
            <option>Washington</option>
        </select>
    <input type="submit" name="dropdown-submit" value="gönder" />
    </form>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.3/js/select2.min.js"></script>
    <script>
        $(document).ready(function() {
            $('.select2').select2();
        });
    </script>
</body>
</html>