Php echo into tag

If you want to wrap an HTML tag around the user display names, you can simply close and open the PHP tags:

<div class="author_sidebar">
  <?php if ( is_singular( 'post' ) ) { ?>
    <div class="foo"><?php echo get_avatar( get_the_author_meta( 'user_email' ), 75 ); ?></div>
    <div class="bar"><?php echo get_the_author_meta( 'display_name' ); ?>
  <?php } ?>
</div>