Need to add class to first element in a foreach loop

Use below code

<?php 

    $isFirst = true;

    $i=1;   

    foreach ( $attachments as $attachment ):
    $description = $attachment->post_content;
    if($i==1)
    {
            echo '<div class="item'.$isFirst.'" data-description="' . $description . '">';
            echo wp_get_attachment_image($attachment->ID, 'large');
            echo '</div>';
    }
    else
    {
            echo '<div class="item" data-description="' . $description . '">';
            echo wp_get_attachment_image($attachment->ID, 'large');
            echo '</div>';
    }
        $i++;
      endforeach;
?>