こちらがコードなんですが、gmailで受信すると件名が文字化けします。どうすればよいでしょうか？

 <?php 

if ($_SERVER["REQUEST_METHOD"] == "POST") {
$name = trim(filter_input(INPUT_POST,"name",FILTER_SANITIZE_STRING));
$email = trim(filter_input(INPUT_POST,"email",FILTER_SANITIZE_EMAIL));
$category = trim(filter_input(INPUT_POST,"category",FILTER_SANITIZE_STRING));
$title = trim(filter_input(INPUT_POST,"title",FILTER_SANITIZE_STRING));
$format = trim(filter_input(INPUT_POST,"format",FILTER_SANITIZE_STRING));
$genre = trim(filter_input(INPUT_POST,"genre",FILTER_SANITIZE_STRING));
$year = trim(filter_input(INPUT_POST,"year",FILTER_SANITIZE_STRING));
$details = trim(filter_input(INPUT_POST,"details",FILTER_SANITIZE_SPECIAL_CHARS));

if ($name == "" || $email == "" || $category == "" || $title == "") {
    $error_message = "Please fill in the required fields: Name, Email, Category and Title";
}
if (!isset($error_message) && $_POST["address"] != "") {
    $error_message = "Bad form input";
}

require("inc/phpmailer/class.phpmailer.php");

$mail = new PHPMailer;

if (!isset($error_message) && !$mail->ValidateAddress($email)) {
    $error_message = "Invalid Email Address";
}

if (!isset($error_message)) {
    $email_body = "";
    $email_body .= "Name " . $name . "\n";
    $email_body .= "Email " . $email . "\n";
    $email_body .= "Suggested Item\n";
    $email_body .= "Category " . $category . "\n";
    $email_body .= "Title " . $title . "\n";
    $email_body .= "Format " . $format . "\n";
    $email_body .= "Genre " . $genre . "\n";
    $email_body .= "Year " . $year . "\n";
    $email_body .= "Details " . $details . "\n";

    $mail->setFrom($email, $name);
    $mail->addAddress('treehouse@localhost', 'Alena');     // Add a recipient

    $mail->isHTML(false);                                  // Set email format to HTML

    $mail->Subject = 'Personal Media Library Suggestion from ' . $name;
    $mail->Body    = $email_body;

    if($mail->send()) {
        header("location:suggest.php?status=thanks");
        exit;
    }
    $error_message = 'Message could not be sent.';
    $error_message .= 'Mailer Error: ' . $mail->ErrorInfo;
}

}

$pageTitle = "Suggest a Media Item";
$section = "suggest";

include("inc/header.php"); 
?>

<div class="section page">
 <div class="wrapper">
    <h1>Suggest a Media Item</h1>
    <?php if (isset($_GET["status"]) && $_GET["status"] == "thanks") {
        echo "<p>Thanks for the email! I&rsquo;ll check out your suggestion shortly!</p>";
    } else {
        if (isset($error_message)) {
            echo "<p class='message'>".$error_message . "</p>";
        } else {
            echo "<p>If you think there is something I&rsquo;m missing, let me know! Complete the form to send me an email.</p>";
        }
    ?>
    <form method="post" action="suggest.php">
        <table>
        <tr>
            <th><label for="name">Name (required)</label></th>
            <td><input type="text" id="name" name="name" /></td>
        </tr>
        <tr>
            <th><label for="email">Email (required)</label></th>
            <td><input type="text" id="email" name="email" /></td>
        </tr>
        <tr>
            <th><label for="category">Category (required)</label></th>
            <td><select id="category" name="category">
                <option value="">Select One</option>
                <option value="Books">Book</option>
                <option value="Movies">Movie</option>
                <option value="Music">Music</option>
            </select></td>
        </tr>
        <tr>
            <th><label for="title">Title (required)</label></th>
            <td><input type="text" id="title" name="title" /></td>
        </tr>
        <tr>
            <th><label for="format">Format</label></th>
            <td><select id="format" name="format">
                <option value="">Select One</option>
                <optgroup label="Books">
                    <option value="Audio">Audio</option>
                    <option value="Ebook">Ebook</option>
                    <option value="Hardback">Hardback</option>
                    <option value="Paperback">Paperback</option>
                </optgroup>
                <optgroup label="Movies">
                    <option value="Blu-ray">Blu-ray</option>
                    <option value="DVD">DVD</option>
                    <option value="Streaming">Streaming</option>
                    <option value="VHS">VHS</option>
                </optgroup>
                <optgroup label="Music">
                    <option value="Cassette">Cassette</option>
                    <option value="CD">CD</option>
                    <option value="MP3">MP3</option>
                    <option value="Vinyl">Vinyl</option>
                </optgroup>
            </select></td>
        </tr>
        <tr>
            <th>
                <label for="genre">Genre</label>
            </th>
            <td>
                <select name="genre" id="genre">
                    <option value="">Select One</option>
                    <optgroup label="Books">
                        <option value="Action">Action</option>
                        <option value="Adventure">Adventure</option>
                        <option value="Comedy">Comedy</option>
                        <option value="Fantasy">Fantasy</option>
                        <option value="Historical">Historical</option>
                        <option value="Historical Fiction">Historical Fiction</option>
                        <option value="Horror">Horror</option>
                        <option value="Magical Realism">Magical Realism</option>
                        <option value="Mystery">Mystery</option>
                        <option value="Paranoid">Paranoid</option>
                        <option value="Philosophical">Philosophical</option>
                        <option value="Political">Political</option>
                        <option value="Romance">Romance</option>
                        <option value="Saga">Saga</option>
                        <option value="Satire">Satire</option>
                        <option value="Sci-Fi">Sci-Fi</option>
                        <option value="Tech">Tech</option>
                        <option value="Thriller">Thriller</option>
                        <option value="Urban">Urban</option>
                    </optgroup>
                    <optgroup label="Movies">
                        <option value="Action">Action</option>
                        <option value="Adventure">Adventure</option>
                        <option value="Animation">Animation</option>
                        <option value="Biography">Biography</option>
                        <option value="Comedy">Comedy</option>
                        <option value="Crime">Crime</option>
                        <option value="Documentary">Documentary</option>
                        <option value="Drama">Drama</option>
                        <option value="Family">Family</option>
                        <option value="Fantasy">Fantasy</option>
                        <option value="Film-Noir">Film-Noir</option>
                        <option value="History">History</option>
                        <option value="Horror">Horror</option>
                        <option value="Musical">Musical</option>
                        <option value="Mystery">Mystery</option>
                        <option value="Romance">Romance</option>
                        <option value="Sci-Fi">Sci-Fi</option>
                        <option value="Sport">Sport</option>
                        <option value="Thriller">Thriller</option>
                        <option value="War">War</option>
                        <option value="Western">Western</option>
                    </optgroup>
                    <optgroup label="Music">
                        <option value="Alternative">Alternative</option>
                        <option value="Blues">Blues</option>
                        <option value="Classical">Classical</option>
                        <option value="Country">Country</option>
                        <option value="Dance">Dance</option>
                        <option value="Easy Listening">Easy Listening</option>
                        <option value="Electronic">Electronic</option>
                        <option value="Folk">Folk</option>
                        <option value="Hip Hop/Rap">Hip Hop/Rap</option>
                        <option value="Inspirational/Gospel">Insirational/Gospel</option>
                        <option value="Jazz">Jazz</option>
                        <option value="Latin">Latin</option>
                        <option value="New Age">New Age</option>
                        <option value="Opera">Opera</option>
                        <option value="Pop">Pop</option>
                        <option value="R&B/Soul">R&amp;B/Soul</option>
                        <option value="Reggae">Reggae</option>
                        <option value="Rock">Rock</option>
                    </optgroup>
                </select>
            </td>
        </tr>
        <tr>
            <th><label for="year">Year</label></th>
            <td><input type="text" id="year" name="year" /></td>
        </tr>
        <tr>
            <th><label for="name">Additional Details</label></th>
            <td><textarea name="details" id="details"></textarea></td>
        </tr>
        <tr style="display:none">
            <th><label for="address">Address</label></th>
            <td><input type="text" id="address" name="address" />
            <p>Please leave this field blank</p></td>
        </tr>
        </table>
        <input type="submit" value="Send" />
    </form>
    <?php } ?>
  </div>
</div>

<?php include("inc/footer.php"); ?>