It is not all that hard;
You table should become;
CREATE TABLE users (
username varchar(30),
zipp varchar(10)
password varchar(32));
(Note that the numberof characters depends on your location)
The insert would become:
$q = "INSERT INTO users VALUES ('$username', '$zippcode', '$password')";
And when the user is logged in you can:
$q = "SELECT * from users where zipp = $ThisUserZipp)";