I've been working on a website lately. It's a pretty small one and most of the content is just entered in html. However, I need one section where the owner can post things. I know how to accomplish this in php, but I'm not sure about how the owner will have access on this "administrator" mode.
Should I build an administrator page where there are options like, "Post new article" or "upload a file"? If so, I must give him this specific link and username and passwords, because he won't be able to navigate to the admin page from the official one. How is this usually done?
You would need to provide a login form to access the admin page. Once you have crated the login form have that submit it to a PHP script that will verify the details submitted with the one held in a database.
If the details match then you could set a session variable, say $_SESSION[ 'admin' ] and when you access the page get it to check that you are logged in, if you do not have the session variable set then redirect the user back to the home page.
You will need o have a look around the net for simple login forms to get you going, once you have a form built you would be able to return here if further questions that people would be able to help with.
To complete this you will need a knowledge of either mysqli or PDO, the second being the preferred option. An understanding of the use of password_hash() and password_verify() as well as a database to store the information.
Good luck