Client Area Login Script

I want to create a "Client" only area that a user will have to supply a User-id and Password to gain access to and to ALL of the web pages within it. If a user does not have a User-id and Password, I want them to be able to Register for one, and if they are a "Client", then I will Activate their access.

My question is: Can someone recommend either a Free or low cost script that will allow me to do this. I would want the script to have an "Admin" function to Activate a user request, Cancel a Users access. I would also want a "User" function that would allow the user to change some of the registration data (Password, Email). I would prefer that the script is PHP.