How to get user IP address in PHP

/** 
* 用来获取用户的ip地址方法 
* @return array|false|string 用户的ip地址 
*/ 
   public function getUserIP() 
   { 
       if(getenv("HTTP_CLIENT_IP") && strcasecmp(getenv("HTTP_CLIENT_IP"), "unknow")){ 
           $ip = getenv("HTTP_CLIENT_IP"); 
       }else if(getenv("HTTP_X_FORWARDED_FOR") && strcasecmp(getenv("HTTP_X_FORWARDED_FOR"), "unknow")){ 
           $ip = getenv("HTTP_X_FORWARDED_FOR"); 
       }else if(getenv("REMOTE_ADDR") && strcasecmp(getenv("REMOTE_ADDR"), "unknow")){ 
           $ip = getenv("REMOTE_ADDR"); 
       }else if(isset($_SERVER["REMOTE_ADDR"]) && $_SERVER["REMOTE_ADDR"] && strcasecmp($_SERVER["REMOTE_ADDR"],"unknow")){ 
           $ip = $_SERVER["REMOTE_ADDR"]; 
       }else{ 
           $ip = "unknow"; 
       } 
       return $ip; 
   }