it's not.
Just thought reading the manual would have been a first good step but...
First you'll have to make sure that you have a database to store the info (or else you'll have to hard code it which is really not the way to go)
Second you create a form in a login page asking for login name and password. In the ation param of the form you specify a PHP script which will :
1. Check for user/pw info in the database (search the previous posts that's a standard question)
2. if it's a valid user initiate a session so you can check if he is recognized on other site pages
3. if he's not recignized as a valid user use a header( "location:login.php") function to redirect him to the login page.
4. To add comments to login page such as "wrong username" etc. use query strings (ie. login.php?error=user). You can access these variables in the PHP code with $_REQUEST[ 'error' ] array element.
Hope it helps and hope you didn't take offense about my little "joke" 😉
If you need more post what you have available as far as DB is concerned and version of PHP too.
GL & HF