Sign up script security check

Hey guys, I have this sign up script and I'm using mysql_real_escape_string .I know prepared statements are safer but I'm just not experienced enough to use them, I just can't figure out how. Anyway here's the script:

<?php

    $username=mysql_real_escape_string($_POST['username']);
    $password=sha1($_POST['password']);
    $password2=sha1($_POST['password_confirmation']);
    $passcheck=$_POST['password'];
    $todo=mysql_real_escape_string($_POST['todo']);
    $email=mysql_real_escape_string($_POST['email']);
    $fname=mysql_real_escape_string($_POST['fname']);
    $lname=mysql_real_escape_string($_POST['lname']);
    $gender=$_POST['gender'];
    $class=$_POST['class'];
    $section=$_POST['section'];



if(isset($todo) and $todo=="post"){

    $status = "OK";
    $msg="";
    }

if(!isset($username) OR strlen($username) <3){
    $msg=$msg."Username should be equal to or more than 3 characters long.<BR/>";
    $status= "NOTOK";
    }                   

if(mysql_num_rows(mysql_query("SELECT username FROM users WHERE username = '$username'"))){
$msg=$msg."Username already exists. Please try another one.<BR/>";
$status= "NOTOK";
}

if(mysql_num_rows(mysql_query("SELECT email FROM users开发者_开发技巧 WHERE email = '$email'"))){
$msg=$msg."E-mail is already in use. Please try again.<BR/>";
$status= "NOTOK";
}                                       


if ( strlen($passcheck) < 3 ){
    $msg=$msg."Password must be more than 3 charactors long.<BR/>";
    $status= "NOTOK";
    }                   

if ( $password <> $password2 ){
    $msg=$msg."Passwords are not identical.<BR/>";
    $status= "NOTOK";
    }                   
if(!eregi("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$", $email)){
    $msg=$msg."The email is not a valid email.<br/>";
    $status="NOTOK";
    }

if($status=="NOTOK"){
    echo '<div class="statusmsg">'.$msg.'<br/><input class="submitButton" type="button" value="Retry" onClick="location.href='."'signup.php'\"></div>";
}
    else {
        $hash = md5( rand(0,1000) );
        $hash = mysql_real_escape_string($hash);
if(mysql_query("insert into users(username,password,email,fname,lname,hash,gender,class,section) values('$username','$password','$email','$fname','$lname','$hash','$gender','$class','$section')")or die (mysql_error ())){
    echo '<div class="statusmsg">Welcome, You have successfully signed up. Please check the verification e-mail sent to you.</div>';
    $to = $email; 
   $subject = 'Signup | Verification'; 
   $message = ' 

        Thanks for signing up! 
            Your account has been created, you can login with the following credentials after you have activated your account by pressing the url below. 

            ------------------------ 
            Username: '.$username.' 
        ------------------------ 

        Please click this link to activate your account: 
   <div id="header">  
         <h3>JMToday > Sign up</h3>  
     </div>         
        http://www.JMtoday.com/verification.php?email='.$email.'&hash='.$hash.' 

   ';

    $headers = 'From:noreply@JMtoday.com' . "\r\n";  
    mail($to, $subject, $message, $headers); 
    }
else { 
echo "Database problem, please contact site admin";
}

}
?>

The user will never see the "database problem" message, as the script will die() out if the query fails. As well, you're embedding HTML into the message, but are not building a proper HTML-format email. Some mail clients may be smart enough to figure out there's HTML and render it as such, but that's just luck.

The hash you generate is limited to generating only 1001 hashes. Given the birthday paradox, after 38 people sign up, the odds of a collision are 50%. After 100 people, the odds are 99.29%. Instead of hashing a random number, do something like:

$hash = md5(serialize($_POST) . $some_other_stuff_in_case_POST_is_empty);