[PHP & SQL] My very first programming thats actually useful

Code:
<?php

//Make connection with the server

$db = new mysqli('localhost', 'testuser14', 'supersecretpassword', 'lottery');

//check if this is a valid mail address

if(isset($_POST['mailform']) && !empty($_POST['mailform'])){

    $email = mysql_real_escape_string($_POST['mailform']);

    if(!mb_ereg("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$", $email)){

        // Wrong mail entered. Will give error message

        print '<script type="text/javascript">';

        print 'alert("Your mail address seems wrong. Please take a look at it.")';

        print '</script>';

    }else{

        // Successful mail

        //Check if the mail address is already registered in the DB

        $mailcheck = mysqli_query($db, "SELECT * FROM maindb WHERE mail = '$email'");

        if(mysqli_num_rows($mailcheck) > 0){

            print '<script type="text/javascript">';

            print 'alert("This mail is already registered!")';

            print '</script>';

        }else{

            // Generate a random number and save it. If the generated number already exists, make a new one.

            $lot1 = mt_rand(1,5300);

            $check = mysqli_query($db, "SELECT * FROM maindb where lotnumber = $lot1");

            if(mysqli_num_rows($check) == 0){

                mysqli_query($db, "INSERT INTO maindb (lotnumber, mail) VALUES ('" . $lot1 . "','" . $email . "');");

                //Send mail

            }else{

                $check2 = $check;

                while(mysqli_num_rows($check2) >= 1){

                    $lot2 = mt_rand(1,5300);

                    $check2 = mysqli_query($db, "SELECT * FROM maindb where lotnumber = $lot2");

                    if(mysqli_num_rows($check2) >= 1){

                        //Lets the while loop repeat itself

                    }else{

                        mysqli_query($db, "INSERT INTO maindb (lotnumber, mail) VALUES ('" . $lot2 . "','" . $email . "');");

                        //Send mail

                    };

                };

            };

        };

    };

};

$db->close();

?>