Add Placeholders in Comment Form

/*
 * Comment Form Placeholder Author, Email, URL
 */
 function placeholder_author_email_url_form_fields($fields) {
    $replace_author = __('Your Name', 'yourdomain');
    $replace_email = __('Your Email', 'yourdomain');
    $replace_url = __('Your Website', 'yourdomain');
    
    $fields['author'] = '<p class="comment-form-author">' . '<label for="author">' . __( 'Name', 'yourdomain' ) . '</label> ' . ( $req ? '<span class="required">*</span>' : '' ) .
                    '<input id="author" name="author" type="text" placeholder="'.$replace_author.'" value="' . esc_attr( $commenter['comment_author'] ) . '" size="20"' . $aria_req . ' /></p>';
                    
    $fields['email'] = '<p class="comment-form-email"><label for="email">' . __( 'Email', 'yourdomain' ) . '</label> ' .
    ( $req ? '<span class="required">*</span>' : '' ) .
    '<input id="email" name="email" type="text" placeholder="'.$replace_email.'" value="' . esc_attr(  $commenter['comment_author_email'] ) .
    '" size="30"' . $aria_req . ' /></p>';
    
    $fields['url'] = '<p class="comment-form-url"><label for="url">' . __( 'Website', 'yourdomain' ) . '</label>' .
    '<input id="url" name="url" type="text" placeholder="'.$replace_url.'" value="' . esc_attr( $commenter['comment_author_url'] ) .
    '" size="30" /></p>';
    
    return $fields;
}
add_filter('comment_form_default_fields','placeholder_author_email_url_form_fields');

/*
 * Comment Form Placeholder Comment Field
 */
 function placeholder_comment_form_field($fields) {
    $replace_comment = __('Your Comment', 'yourdomain');
     
    $fields['comment_field'] = '<p class="comment-form-comment"><label for="comment">' . _x( 'Comment', 'noun' ) .
    '</label><textarea id="comment" name="comment" cols="45" rows="8" placeholder="'.$replace_comment.'" aria-required="true"></textarea></p>';
    
    return $fields;
 }
add_filter( 'comment_form_defaults', 'placeholder_comment_form_field' );