Notice: Undefined index: HTTP_REFERER

Array
(
    [HTTP_HOST] => 192.168.1.10
    [HTTP_USER_AGENT] => Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:15.0) Gecko/20100101 Firefox/15.0
    [HTTP_ACCEPT] => text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
    [HTTP_ACCEPT_LANGUAGE] => en-us,en;q=0.5
    [HTTP_ACCEPT_ENCODING] => gzip, deflate
    [HTTP_CONNECTION] => keep-alive
    [PATH] => /sbin:/usr/sbin:/bin:/usr/bin
    [SERVER_SIGNATURE] => Apache/2.2.3 (CentOS) Server at 192.168.1.10 Port 80
    [SERVER_SOFTWARE] => Apache/2.2.3 (CentOS)
    [SERVER_NAME] => 192.168.1.10
    [SERVER_ADDR] => 192.168.1.10
    [SERVER_PORT] => 80
    [REMOTE_ADDR] => 192.168.1.77
    [DOCUMENT_ROOT] => /var/www/html
    [SERVER_ADMIN] => root@localhost
    [SCRIPT_FILENAME] => /var/www/html/sandeep/test/hash.php
    [REMOTE_PORT] => 53851
    [GATEWAY_INTERFACE] => CGI/1.1
    [SERVER_PROTOCOL] => HTTP/1.1
    [REQUEST_METHOD] => GET
    [QUERY_STRING] =>
    [REQUEST_URI] => /sandeep/test/hash.php
    [SCRIPT_NAME] => /sandeep/test/hash.php
    [PHP_SELF] => /sandeep/test/hash.php
    [REQUEST_TIME] => 1347365919
)

Notice: Undefined index: HTTP_REFERER in /path/to/filename.php

if(isset($_SERVER['HTTP_REFERER'])) {
      echo $_SERVER['HTTP_REFERER'];
   }

echo isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : '';

$server = if(isset($_SERVER['HTTP_REFERER']))? $_SERVER['HTTP_REFERER'] : "";