Premade scripts are for Javascript weenies 🙂 Anyway, this really isn't the sort of thing you can build by slapping together existing code.
It sounds like you do know where to start - you've already defined the requirements, and they sound accurate and reasonable to me.
Do you have PHP, Apache, and MySQL running on your workstation?
Start by designing and creating your database tables. In addition to the table that will be populated by the data entry form, you'll need a table to store user records (with columns for username, password, probably email address, and anything else you want to keep track of, such as last login date).
Then build login and registration pages. You can use either sessions or cookies to recognize visitors, but there are security considerations to be aware of. Unless your site contains particularly sensitive data, though, you can probably ignore them for your first project.
On any pages that should be restricted to logged-in users, you'll want to include a small PHP script at the top, called checklogin.php or something. The code in this file will check the user's cookie or session variables (whichever you're using) and make sure they're logged in. If they aren't, it will redirect them to the login page (using the header() directive).
Now you have a security framework, and you can start on the application proper. The data entry form is probably the best place to start, so you can populate the table with some test data, and so the search page will have something to find.
Let us know if you have any specific questions as you go.
