获得用户的真实IP地址

<?php 
  
/**
 * 获得用户的真实IP地址
 *
 * @return  string
 */ 
function real_ip(){ 
    static $realip = NULL; 
  
    if ($realip !== NULL){ 
        return $realip; 
    } 
  
    if (isset($_SERVER)){ 
        if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])){ 
            $arr = explode(',', $_SERVER['HTTP_X_FORWARDED_FOR']); 
            /* 取X-Forwarded-For中第一个非unknown的有效IP字符串 */ 
            foreach ($arr AS $ip){ 
                $ip = trim($ip); 
  
                if ($ip != 'unknown'){ 
                    $realip = $ip; 
  
                    break; 
                } 
            } 
        } 
        elseif (isset($_SERVER['HTTP_CLIENT_IP'])){ 
            $realip = $_SERVER['HTTP_CLIENT_IP']; 
        } 
        else{ 
            if (isset($_SERVER['REMOTE_ADDR'])){ 
                $realip = $_SERVER['REMOTE_ADDR']; 
            } 
            else{ 
                $realip = '0.0.0.0'; 
            } 
        } 
    } 
    else{ 
        if (getenv('HTTP_X_FORWARDED_FOR')){ 
            $realip = getenv('HTTP_X_FORWARDED_FOR');