在引导导航栏中显示 PHP 变量

我想在登录或注册后使用函数链接((在导航栏中显示用户名，但保存用户名的变量不会显示，仅显示"$can";

 function links()
{
if(!isset($_SESSION['username']))
     {
         echo '         <div class="collapse navbar-collapse" id="signup">
            <!--<form class="navbar-form navbar-right" role="search">
              <div class="form-group">
                <input type="text" class="form-control" placeholder="Search">
              </div>
              <button type="submit" class="btn btn-default">Submit</button>
            </form>-->
          <ul class="nav navbar-nav navbar-right" id="signup">
            <li><a href="signup.php"><span class="glyphicon glyphicon-user"></span> Sign Up</a></li>
            <li><a href="index.php"><span class="glyphicon glyphicon-log-in"></span> Login</a></li>
          </ul>
        </div>';
         }
         else
         {
           $acn= $_SESSION['username'];
        echo   '<div class="collapse navbar-collapse" id="signup">
          <ul class="nav navbar-nav navbar-right" id="signup">
            <li><a href='"account.php'"> $acn </a></li>
            <li><a href="logout.php"><span class="glyphicon glyphicon-log-out"></span> Log Out</a></li>
          </ul>
        </div>';
    }
}

函数如下所示

<nav class="navbar navbar-inverse" role="navigation">
    <div class="container-fluid">
        <div class="navbar-header">
          <a class="navbar-brand" href="#">OOOO</a>
            <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#signup">
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
        </div>
        <?php echo links(); ?>
    </div>
</nav>

请问我如何实现这一点，谢谢。