PHP中未定义的变量，我做错了什么？

I've been trying to get this PHP script to work, but I can't seem to have it work. :\

<?php function getBrowser() {
  $u_agent = $_SERVER['HTTP_USER_AGENT'];
  $ub = '';
  if(preg_match('/MSIE/i',$u_agent)) {
    $ub = "Internet Explorer";
  }
  elseif(preg_match('/Firefox/i',$u_agent)) {
    $ub = "Mozilla Firefox";
  }
  elseif(preg_match('/Safari/i',$u_agent)) {
    $ub = "Apple Safari";
  }
  elseif(preg_match('/Chrome/i',$u_agent)) {
    $ub = "Google Chrome";
  }
  elseif(preg_match('/Opera/i',$u_agent)) {
    $ub = "Opera";
  }
  elseif(preg_match('/Netscape/i',$u_agent)) {
    $ub = "Netscape";
  }
  return $ub;
}
echo $ub;
?>

I am just trying to make PHP detect browsers correctly, but I always get, "Undefined Variable: ub" at "echo $ub;"

What am I doing wrong this time?

$ub is defined within a function and then returned. To get the value of $ub, you need to call the function:

echo getBrowser();