Laravel 5.1 Blade（或只是普通回声）错误地显示类

I have the following code in my Laravel (5.1) view. The code in the PHP block decides which class I should apply to the first td element below.

@foreach($datesUnavailable as $dateUnavailable)
    <?php
        $unavailableDate = strtotime($dateUnavailable->unavailable_on);
        $creationDate = strtotime($dateUnavailable->created_at);
        $newformat = date('D, M d, Y',$unavailableDate);

        $createdBefore = ($unavailableDate - $creationDate)/86400;

        if($createdBefore >= 5) {
            $class = "alert alert-success";                                           
        } else if ($createdBefore < 5 && $createdBefore >= 3) {
            $class = "alert alert-warning"; 
        } else {
            $class = "alert alert-danger";
        }
    ?>
    <tr>
        <td class={{ $class }}>{{ $newformat }}</td>
        <td>{{ $dateUnavailable->unavailability_type }}</td>
        <td>
            {!! Form::open([
                'method' => 'DELETE',
                'route' => ['delete-unavailability', $alias, $dateUnavailable->uc_key],
                'onsubmit' => 'return submitResult();'
            ]) !!}
                {!! Form::submit('Delete', ['class' => 'btn btn-danger btn-xs']) !!}
            {!! Form::close() !!}
        </td>
    </tr>
@endforeach

However, when the page renders, the classes on the td element show as

<td class="alert" alert-success="">Wed, Feb 28, 2018</td>

That is, I want it to show class="alert alert-success" (based on the logic) but it displays this weird string. If I do a var_dump($class), I see the value of the $class variable as "alert alert-success".

Any idea why this might be happening?