in another script
$username = "Username";
$password = "Password";
// Do sql stuffs here
$result = mysql_result($mysqlStuffs);
The point that the inclusion is called the values are now available. Unsetting the username and password results in neither being accessible outside of the called script.
If sql is used commonly, which it often is, you may want to consider scripting an object for it, preferably a php5 object if you have php 5 available, however, the php 4 engine would do. This way the sql connection can be established in multiple scripts without needing to reconstruct the entire call every time.