<div class="avatar">
  <?php if (isset($_SESSION['access_token']))
  { echo "<a href='userfeed.php?uid={$uid}&{$username}'><img src="{$avatar}">; }
  else
  { echo "<img src="{$avatar}">; } ?>
  </div>

syntax error, unexpected T_ENCAPSED_AND_WHITESPACE

echo "<a href='userfeed.php?uid={$uid}&{$username}'><img src="{$avatar}">;

<?php if (isset($_SESSION['access_token'])) {echo'<a href="userfeed.php?uid={$uid}&{$username}"> <img src="{$avatar}"></a>';} else { echo '<img src="{$avatar}>'; } ?>