Well, I think you might be doing this backwards a bit ;-)
In fact, I woudl never use session_register for security resons (won't go into that). Instead, use the superglobals to set and get values from the session variable.
On every page, just throw in session_start() before writing any headers.
Then, if you want to set a session variable in one script, use:
$_SESSION["my_variable"] = "some value";
So, in another page load when you want to use that variable again for a SQL query:
$sql = "SELECT * FROM some_table WHERE some_field = '" . $_SESSION["my_variable"] . "'";