Do you just want to keep the user logged in on your site while he/she goes from page to page? You could store that info into either a session variable or a cookie. If you do store passwords, always hash them (in the DB and session var or cookie), either using MD5 or there are others as well with more complex encryption.
If you absolutely need to have the login script displayed in parts on each page, then as NogDog mentioned you could include a template file in each page, with arguments, like:
Then parse the arguments within the 'file.php' file.
See here Example #3
Always remember to strip out potentially dangerous data from inputs ($POST and $GET). Use the mysql_real_escape_string() function to do so.
Look up a hash value, convert timestamps or convert numeral bases.