(Guitaristka @ 6.1.2019, 01:11)

RewriteEngine on 
RewriteCond %{HTTP_REFERER} !^$ 
RewriteCond %{HTTP_REFERER} !^http://guitaristka\.ru [NC] 
RewriteRule ^.*$ /go.php [NC,QSA]

if($_SERVER['HTTP_HOST'] == parse_url($_SERVER["HTTP_REFERER"], PHP_URL_HOST)) {
   header('Location: '.$_GET['go']);
}

     if($_SERVER['HTTP_HOST'] == parse_url($_SERVER["HTTP_REFERER"], PHP_URL_HOST)) 
	 {
       header('Location: '.$_GET['go']);
     }

http://www.guitaristka.ru/go.php?go=https://yandex.ru/

<?php
  if(isset($_SERVER["HTTP_REFERER"])) {
    if($_SERVER['HTTP_HOST'] == parse_url($_SERVER["HTTP_REFERER"], PHP_URL_HOST)) {
      header('Location: '.$_GET['go']);
     }
   } else {
    // Выводим ошибку 404
    header("HTTP/1.0 404 Not Found");
    exit;
  }

<?php
echo '<br>HTTP_REFERER:';
var_dump($_SERVER["HTTP_REFERER"]);

echo '<br> "'.$_SERVER['HTTP_HOST'].'" == "'. parse_url($_SERVER["HTTP_REFERER"], PHP_URL_HOST).'"';
echo '<pre>Информация по заголовкам сервера:';
var_dump($_SERVER);

 <?php
      if(isset($_SERVER["HTTP_REFERER"])) {
        if($_SERVER['HTTP_HOST'] == parse_url($_SERVER["HTTP_REFERER"], PHP_URL_HOST)) {
          header('Location: '.$_GET['go']);
         }
       } else {
        // Выводим ошибку 404
        header("HTTP/1.0 404 Not Found");
        exit;
      }
	  
	  echo '<br>HTTP_REFERER:';
var_dump($_SERVER["HTTP_REFERER"]);
echo '<br> "'.$_SERVER['HTTP_HOST'].'" == "'. parse_url($_SERVER["HTTP_REFERER"], PHP_URL_HOST).'"';
echo '<pre>Информация по заголовкам сервера:';
var_dump($_SERVER);

?>

Warning: Cannot modify header information - headers already sent by (output started at /home/guitaristes/public_html/go.php:2) in /home/guitaris/public_html/go.php on line 5

HTTP_REFERER:string(36) "http://www.guitaristka.ru/main.shtml"
"www.guitaristka.ru" == "www.guitaristka.ru"

ааНбаОбаМаАбаИб аПаО аЗаАаГаОаЛаОаВаКаАаМ баЕбаВаЕбаА:array(40) {
  ["SERVER_SIGNATURE"]=>
  string(0) ""
  ["UNIQUE_ID"]=>
  string(24) "XDG231ttyXcAAB6I0jcAAAAZ"
  ["HTTP_X_FORWARDED_FOR"]=>
  string(12) "46.242.10.91"
  ["HTTP_USER_AGENT"]=>
  string(77) "Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:64.0) Gecko/20100101 Firefox/64.0"
  ["PHPRC"]=>
  string(26) "/home/guitaristes/public_html"
  ["SERVER_PORT"]=>
  string(2) "80"
  ["HTTP_HOST"]=>
  string(18) "www.guitaristka.ru"
  ["HTTP_X_REAL_IP"]=>
  string(12) "46.242.10.91"
  ["DOCUMENT_ROOT"]=>
  string(26) "/home/guitaristes/public_html"
  ["PHP_DOCUMENT_ROOT"]=>
  string(14) "/home/guitaris"
  ["SCRIPT_FILENAME"]=>
  string(33) "/home/guitaris/public_html/go.php"
  ["REQUEST_URI"]=>
  string(35) "/go.php?go=http://www.musiclady.ru/"
  ["SCRIPT_NAME"]=>
  string(7) "/go.php"
  ["HTTP_CONNECTION"]=>
  string(5) "close"
  ["PHP_FCGI_CHILDREN"]=>
  string(1) "0"
  ["REMOTE_PORT"]=>
  string(5) "55510"
  ["PATH"]=>
  string(13) "/bin:/usr/bin"
  ["PWD"]=>
  string(26) "/home/guitaristes/public_html"
  ["SERVER_ADMIN"]=>
  string(24) "webmaster@guitaristka.ru"
  ["REDIRECT_STATUS"]=>
  string(3) "200"
  ["TZ"]=>
  string(13) "Europe/Moscow"
  ["HTTP_ACCEPT_LANGUAGE"]=>
  string(35) "ru-RU,ru;q=0.8,en-US;q=0.5,en;q=0.3"
  ["HTTP_REFERER"]=>
  string(36) "http://www.guitaristka.ru/main.shtml"
  ["HTTP_ACCEPT"]=>
  string(63) "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"
  ["REMOTE_ADDR"]=>
  string(12) "46.242.10.91"
  ["SHLVL"]=>
  string(1) "0"
  ["SERVER_NAME"]=>
  string(18) "www.guitaristka.ru"
  ["SERVER_SOFTWARE"]=>
  string(6) "Apache"
  ["MAGICK_THREAD_LIMIT"]=>
  string(1) "2"
  ["QUERY_STRING"]=>
  string(27) "go=http://www.musiclady.ru/"
  ["SERVER_ADDR"]=>
  string(14) "91.109.201.119"
  ["GATEWAY_INTERFACE"]=>
  string(7) "CGI/1.1"
  ["HTTP_UPGRADE_INSECURE_REQUESTS"]=>
  string(1) "1"
  ["SERVER_PROTOCOL"]=>
  string(8) "HTTP/1.0"
  ["REQUEST_METHOD"]=>
  string(3) "GET"
  ["HTTP_COOKIE"]=>
  string(579) "last_visit=1546751149240::1546761949240; phpbb3_ginr1_k=se8uce5pwefvp1tw; _ym_uid=1517900361216872457; coppermine_data=YTo0OntzOjI6IklEIjtzOjMyOiI0Y2M4MDY5MGVmYjliOGNkMTk1NzA0MGQ2ZTQy
NTdlZiI7czoyOiJhbSI7aToxO3M6NToibGl2X2EiO2E6NDp7aTowO2k6NTtpOjE7aTozO2k6MjtpOjE7a
TozO2k6NDt9czozOiJsaXYiO2E6NTp7aTowO3M6NDoiMzU1NiI7aToxO3M6NDoiMzg0NSI7aToyO3M6ND
oiMzc2MiI7aTozO3M6MToiMSI7aTo0O3M6MzoiMjM2Ijt9fQ%3D%3D; 0dd0220d4d5cd6e0798559ab5508fb7d=57c7f1d7f6ab1cb824770ebf8bc0ac94; phpbb3_ginr1_u=5; phpbb3_ginr1_sid=a1a5b961c1ca9fbf152aad6f3aad3601; PHPSESSID=6d482763e6261ca9426bd9d349fb254f"
  ["PHP_SELF"]=>
  string(7) "/go.php"
  ["REQUEST_TIME"]=>
  int(1546761951)
  ["argv"]=>
  array(1) {
    [0]=>
    string(27) "go=http://www.musiclady.ru/"
  }
  ["argc"]=>
  int(1)
}

Warning: Cannot modify header information - headers already sent by (output started at /home/guitaris/public_html/go.php:2) in /home/guitaris/public_html/go.php on line 9

    <?php
      if(isset($_SERVER["HTTP_REFERER"])) {
        if($_SERVER['HTTP_HOST'] == parse_url($_SERVER["HTTP_REFERER"], PHP_URL_HOST)) {
          header('Location: '.$_GET['go']);
         }
       } else {
        // Выводим ошибку 404
        header("HTTP/1.0 404 Not Found");
        exit;
      }

Warning: Cannot modify header information - headers already sent by (output started at /home/guitaristes/public_html/go.php:2) in /home/guitaris/public_html/go.php on line 5

Warning: Cannot modify header information - headers already sent by (output started at /home/guitaristes/public_html/go.php:2) in /home/guitaris/public_html/go.php on line 5

<?php
    if(isset($_SERVER["HTTP_REFERER"])) {
        if($_SERVER['HTTP_HOST'] == parse_url($_SERVER["HTTP_REFERER"], PHP_URL_HOST)) {
          header('Location: '.$_GET['go']);
         }
       } else {
        // Выводим ошибку 404
        header("HTTP/1.0 404 Not Found");
        exit;
      }
?>

(Guitaristka @ 6.1.2019, 15:53)

<?php
    if(isset($_SERVER["HTTP_REFERER"])) {
        if($_SERVER['HTTP_HOST'] == parse_url($_SERVER["HTTP_REFERER"], PHP_URL_HOST)) {
          header('Location: '.$_GET['go']);
         }
       } else {
        header('Location: http://site.ru');
        exit;
      }
?>

(Ilya74 @ 6.1.2019, 01:04)

<?php
  ob_start();
  if (isset($_SERVER['HTTP_REFERER'])) {
    if ($_SERVER['HTTP_HOST'] == parse_url($_SERVER['HTTP_REFERER'], PHP_URL_HOST)) {
      header('Location: '.$_GET['go']);
    }
  } else {
    header('Location: http://site.ru');
    exit;
  }
  ob_end_flush();
?>

header('Location: http://site.ru');