OR the field isn't called sender. Use phpMyAdmin or NaviCat (or any other mySQL program) to check out how you named your database.
You can also include at the top, or rather in the config error_reporting(E_ALL); or maybe error_reporting(E_ALL ^ E_NOTICE);. This way you can see all the errors except the notices (like define and such). I use it mainly for developing since then I can see if my query is wrong, my fetch didn't go as planned or maybe if I even haven't established a connection to the database.