about PHP's filter_var function

[lx <lxlenovostar@xxxxxxxxx>]

Hello:
       I want to use filter_var function by this way:

                $ip = "192.168.0.1";

                if( !filter_var($ip, FILTER_VALIDATE_IP) )
                {
                        echo "IP is not valid";
                }
                else
                {
                         echo "IP is valid";
                }

I want to check the string $ip is IP address or not.but my PHP version is
5.1.6.
 and I know the filter_var requires at least PHP version 5.2.0.
so, Any other function in PHP 5.1.6 can slove this work and replace the
filter_var function ?

Thank you, I'm a new one, so I don't know much about  PHP documentation.

By the way, The PHP version is required. so I can't upgrade it.