Detailed explanation of several methods of obtaining user access IP address in PHP

<?php                                                                 //方法1：
$ip = $_SERVER["REMOTE_ADDR"];
echo $ip;
 
//方法2：
$user_IP = ($_SERVER["HTTP_VIA"]) ? $_SERVER["HTTP_X_FORWARDED_FOR"] : $_SERVER["REMOTE_ADDR"];
$user_IP = ($user_IP) ? $user_IP : $_SERVER["REMOTE_ADDR"];
echo $user_IP;
 
//方法3：
function getRealIp()
{
  $ip=false;
  if(!empty($_SERVER["HTTP_CLIENT_IP"])){
    $ip = $_SERVER["HTTP_CLIENT_IP"];
  }
  if (!empty($_SERVER[&#39;HTTP_X_FORWARDED_FOR&#39;])) {
    $ips = explode (", ", $_SERVER[&#39;HTTP_X_FORWARDED_FOR&#39;]);
    if ($ip) { array_unshift($ips, $ip); $ip = FALSE; }
    for ($i = 0; $i < count($ips); $i++) {
      if (!eregi ("^(10│172.16│192.168).", $ips[$i])) {
        $ip = $ips[$i];
        break;
      }
    }
  }
  return ($ip ? $ip : $_SERVER[&#39;REMOTE_ADDR&#39;]);
}
echo getRealIp();
 
//方法4：
if ($HTTP_SERVER_VARS["HTTP_X_FORWARDED_FOR"])
{
  $ip = $HTTP_SERVER_VARS["HTTP_X_FORWARDED_FOR"];
}
elseif ($HTTP_SERVER_VARS["HTTP_CLIENT_IP"])
{
  $ip = $HTTP_SERVER_VARS["HTTP_CLIENT_IP"];
}
elseif ($HTTP_SERVER_VARS["REMOTE_ADDR"])
{
  $ip = $HTTP_SERVER_VARS["REMOTE_ADDR"];
}
elseif (getenv("HTTP_X_FORWARDED_FOR"))
{
  $ip = getenv("HTTP_X_FORWARDED_FOR");
}
elseif (getenv("HTTP_CLIENT_IP"))
{
  $ip = getenv("HTTP_CLIENT_IP");
}
elseif (getenv("REMOTE_ADDR"))
{
  $ip = getenv("REMOTE_ADDR");
}
else
{
  $ip = "Unknown";
}
echo $ip ;
 
//方法5：
if(getenv(&#39;HTTP_CLIENT_IP&#39;)) {
  $onlineip = getenv(&#39;HTTP_CLIENT_IP&#39;);
} elseif(getenv(&#39;HTTP_X_FORWARDED_FOR&#39;)) {
  $onlineip = getenv(&#39;HTTP_X_FORWARDED_FOR&#39;);
} elseif(getenv(&#39;REMOTE_ADDR&#39;)) {
  $onlineip = getenv(&#39;REMOTE_ADDR&#39;);
} else {
  $onlineip = $HTTP_SERVER_VARS[&#39;REMOTE_ADDR&#39;];
}
echo $onlineip;