Checkbox数组的未定义索引

Question

I am sending an array of checkbox values from a Form to my controller.
I get no value when I try to access them using $_Post. So I decided to try using var_dump(). The result is that I see no value inside my array. If i check 3 checkboxes I get:

array(3) { [0]=> string(0) "" [1]=> string(0) "" [2]=> string(0) "" }

I don't understand how to correct this error.

-- My form --

<div class="container">
<div>
    <form class="form-horizontal" id="form" action="/MVC/home/languageChosen" method="post">
        <div class="form-group">
            <table>
                <tr>
                    <th>Language</th>
                    <th>Learn</th>
                    <th>Teach</th>
                </tr>
                <?php foreach ($data['languageData'] as $languageData): ?>
                    <tr>
                        <td><?php echo $languageData['language_name'] ?></td>
                        <td><input type="checkbox" name="learn[]" value="<?php $languageData['language_id'] ?>"></td>
                        <td><input type="checkbox" name="teach[]" value="<?php $languageData['language_id'] ?>"></td>
                    </tr>
                <?php endforeach ?>
            </table>
            <button id="continueButton" name="action" type="submit" class="btn btn-default" value="RegisterLanguage">Continue</button>
        </div>
    </form>
</div>

-- Controller --

public function languageChosen()
{
    echo "You posted: " . implode(',', $_POST['learn']); // This results in: You posted: ,,
    var_dump($_POST['learn']);
    die();
}

EDIT

When I echo $languageData['language_id'], it results in the image below.