PHP Register Script

<?php
$con = mysql_connect('localhost', 'root', '') OR die("Error: ".mysql_error());
mysql_select_db('db_name', $con) OR die("Error: ".mysql_error());

function protect($string){
    $string = mysql_real_escape_string($string);
    $string = strip_tags($string);
    $string = addslashes($string);

    return $string;
}
if(!$_POST['submit']){
    echo "<table border=\"0\" cellspacing=\"3\" cellpadding=\"3\" align=\"center\">\n";
    echo "<form method=\"post\" action=\"".$self."\">\n";
    echo "<tr><td colspan=\"2\" align=\"center\" bgcolor=\"#333333\"><font color=\"#ffffff\">Registration Form</font></td></tr>\n";
    echo "<tr><td>Username</td><td><input type=\"text\" name=\"username\"></td></tr>\n";
    echo "<tr><td>Password</td><td><input type=\"password\" name=\"password\"></td></tr>\n";
    echo "<tr><td>Confirm</td><td><input type=\"password\" name=\"passconf\"></td></tr>\n";
    echo "<tr><td>E-Mail</td><td><input type=\"text\" name=\"email\"></td></tr>\n";
    echo "<tr><td align=\"left\"><input type=\"submit\" name=\"submit\" value=\"Register\"></form></td><form action='login.php' method='POST'><td align=\"right\"><input type=\"submit\" value=\"Login\"></td></tr>\n";
    echo "</form></table>\n";
}else {
    $username = protect($_POST['username']);
    $password = protect($_POST['password']);
    $confirm = protect($_POST['passconf']);
    $email = protect($_POST['email']);

    $errors = array();

        if(!$username){
            $errors[] = "Username is not defined!";
        }

        if(!$password){
            $errors[] = "Password is not defined!";
        }

        if($password){
            if(!$confirm){
                $errors[] = "Confirmation password is not defined!";
            }
        }

        if(!$email){
            $errors[] = "E-mail is not defined!";
        }



        if($username){
            if(!ctype_alnum($username)){
                $errors[] = "Username can only contain numbers and letters!";
            }

            $range = range(1,32);
            if(!in_array(strlen($username),$range)){
                $errors[] = "Username must be between 1 and 32 characters!";
            }
        }

        if($password && $confirm){
            if($password != $confirm){
                $errors[] = "Passwords do not match!";
            }
        }

        if($email){
            $checkemail = "/^[a-z0-9]+([_\\.-][a-z0-9]+)*@([a-z0-9]+([\.-][a-z0-9]+)*)+\\.[a-z]{2,}$/i";
            if(!preg_match($checkemail, $email)){
                $errors[] = "E-mail is not valid, must be name@server.tld!";
            }
        }


        if($username){
            $sql = "SELECT * FROM `users` WHERE `username`='".$username."'";
            $res = mysql_query($sql) or die(mysql_error());

                if(mysql_num_rows($res) > 0){
                    $errors[] = "The username you supplied is already in use!";
                }
        }

        if($email){
            $sql2 = "SELECT * FROM `users` WHERE `email`='".$email."'";
            $res2 = mysql_query($sql2) or die(mysql_error());

                if(mysql_num_rows($res2) > 0){
                    $errors[] = "The e-mail address you supplied is already in use of another user!";
                }

        }

        if(count($errors) > 0){
            foreach($errors AS $error){
                echo $error . "<br>\n";
            }
        }else {
            $sql4 = "INSERT INTO `users`
                    (`username`,`password`,`email`)
                    VALUES ('".$username."','".$password."','".$email."')";
            $res4 = mysql_query($sql4) or die(mysql_error());
            echo "<font align=\"center\"><br><br>You have successfully registered with the username <b>".$username."</b> and the password <b>".$password."</b>!</font>";
        }
}