It is possible to write a login/password system without using a database, if the number of unique logins is small and changes are infrequent. You can do this in PHP with an external text file of logins and password pairs, preferably encrypted, or with them directly stored in an array in the web page.
However, I'm not sure what you mean by the "admin" page? Are you referring to the ftp access to the site? In which case login and password protection would normally already be set up by the web host. Or perhaps to the admin page of a CMS like WordPress? If so, what you can, or need, to do is dependent on the CMS.