php如何获得访问局域网内的服务器的用户的真实ip，即外网ip。

/**
* 获取客户端IP地址
*/
static public function getClientIP() {
static $ip = NULL;
if ( $ip !== NULL )
return $ip;
if ( isset( $_SERVER['HTTP_X_FORWARDED_FOR'] ) ) {
$arr = explode( ',', $_SERVER['HTTP_X_FORWARDED_FOR'] );
$pos = array_search( 'unknown', $arr );
if ( false !== $pos )
unset( $arr[$pos] );
$ip = trim( $arr[0] );
} elseif ( isset( $_SERVER['HTTP_CLIENT_IP'] ) ) {
$ip = $_SERVER['HTTP_CLIENT_IP'];
} elseif ( isset( $_SERVER['REMOTE_ADDR'] ) ) {
$ip = $_SERVER['REMOTE_ADDR'];
}
// IP地址合法验证
$ip = ( false !== ip2long( $ip ) ) ? $ip : '';
return $ip;
}