PHP+MYSQL headache

i have 2 files.
register.php and login.php

LOGIN.PHP
[PHP]
$username = mysql_real_escape_string($_POST["username"]);
$password = mysql_real_escape_string(md5($_POST));

$query = "SELECT * FROM user WHERE username = '$username' AND password = '$password'";

$result = mysql_fetch_array(mysql_query($query));


[/PHP]


NOW i wanna make another one lets say reg2.php
which will update the table with the score for the logged in person !
i cant just do where username = $username etc because all this file will be taking in will be the score [for example from a textfeild]

will i have to do this within login?
if so how?

anyone give me simple session example using my code?

thank you