用户代理的PHP错误

 <?php
   $os = $_SERVER['HTTP_USER_AGENT'];
   echo $os();
 ?>

This code does not work at all, but echo $_SERVER['HTTP_USER_AGENT']; does work.

EDIT: Please ignore this, I fixed it with print_r();

 <?php
 $os = $_SERVER['HTTP_USER_AGENT'];
 echo $os;
 ?>

$os is a variable not a function