Reorder and add Placeholders for Comment Form Fields

<?php
/**
 * Customize comment form
 *
 * @param array $fields Comment form fields.
 */
function wd_customize_comment_form_fields( $fields ) {

	// get value of fields and save as variable
	$author = $fields['author'];
	$email = $fields['email'];
	$comment = $fields['comment'];
	$url = $fields['url'];
	
     // unset other fields to allow reordering
     unset( $fields['url'] );
     unset( $fields['comment'] );
     unset( $fields['author'] );
     unset( $fields['email'] );

     // set fields in proper order with placeholder
     $fields['author'] = $author;
	 $fields['email'] = $email;
	 $fields['comment'] = $comment;
	$fields['url'] = $url;
	
	return $fields;
}
add_filter( 'comment_form_fields', 'wd_customize_comment_form_fields' );

This snippet includes adding placeholder text

<?php
/**
 * Customize comment form
 *
 * @param array $fields Comment form fields.
 */
function wd_customize_comment_form_fields( $fields ) {

     // remove URL field
     unset( $fields['url'] );

     // unset other fields to allow reordering
     unset( $fields['comment'] );
     unset( $fields['author'] );
     unset( $fields['email'] );

     // create placeholder variables
     $comment_placeholder = __( 'Comment', 'slug' );
     $author_placeholder = __( 'Name', 'slug' );
     $email_placeholder = __( 'Email', 'slug' );

     // set fields in proper order with placeholder
     $fields['author'] = '<p class="comment-form-author">' . '<label for="author">' . $author_placeholder . '</label> ' . ( $req ? '<span class="required">*</span>' : '' ) . '<input id="author" name="author" type="text" placeholder="' . $author_placeholder . '" value="' . esc_attr( $commenter['comment_author'] ) . '" size="20"' . $aria_req . ' /></p>';

     $fields['email'] = '<p class="comment-form-email"><label for="email">' . $email_placeholder . '</label> ' . ( $req ? '<span class="required">*</span>' : '' ) . '<input id="email" name="email" type="text" placeholder="' . $email_placeholder . '" value="' . esc_attr(  $commenter['comment_author_email'] ) . '" size="30"' . $aria_req . ' /></p>';

     $fields['comment'] = '<p class="comment-form-comment"><label for="comment">' . _x( 'Comment', 'noun' ) . '</label><textarea id="comment" name="comment" cols="45" rows="8" placeholder="' . $comment_placeholder . '" aria-required="true"></textarea></p>';

	return $fields;
}
add_filter( 'comment_form_fields', 'wd_customize_comment_form_fields' );