Programming languages are always easiest to be learned by doing projects.
Start small, and slowly work your way up. Register and login script is a huge project, so let's put that off for a while.
Start with something as simple as Hello World program.
Then move to something more complicated, but skip over the MySQL things first.
Let's say, something like entering form data in one page, and displaying it in another.
Once you're more comfortable with doing PHP stuffs, start looking into MySQL.
It's easier to learn how to read data, than to enter data. So, use something like PHPMyAdmin to fake your data entry, then use PHP to read and print out their data.
After that, try to make pages where you can enter data on one page, store it in MySQL, and read it on another page.
After that, you can start working on bigger projects, and you can start working on your register/login page.
-stndn.