Not too difficult. Just 3 small steps.
1.) Verify that the [man]strlen/man of the mobile phone is the expected length (10 for US numbers).
2.) Do a SELECT query matching the mobile number:
[indent]
SELECT `id` FROM `tablename` WHERE `mobile_phone` = '{$_GET[mobile_phone]}' LIMIT 1
[/indent]
3.) If 2 doesn't return a valid ID, then show the form, otherwise add it to the INSERT statement:
[indent]
INSERT INTO `tablename` (`column`, `column`, `referrer_id`) VALUES ('{$_POST[var]}', '$_POST[var]}', '{$row[id]}');
[/indent]
Hope that helps.
