php 获取客户端IP地址

// 获取客户端IP地址
function get_client_ip()
{
    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 : '0.0.0.0';
    return $ip;
}