SQL is the name of the language used to interact with a DBMS (Database Management System). The most popular DBMS to be used with PHP is MySQL, so will probably want to get that installed if you haven't already.
User management is almost always done with a combination of PHP and a DBMS. User details are stored in a database table, and PHP authenticates the supplied login details against these user details.
I will say as a word of warning, user management systems tend to be an area where new programmers leave (sometimes massive) security holes. Please don't do the same as most people and go into this project thinking "if I can get it so it works I'll be happy". If it's not secure, it's broken.
If you've just started you will first want to get the PHP basics sorted. Areas you probably want to look at are (securely) interacting with a MySQL database, string hashing, sessions, maybe cookies. Once you've done that (or even before), have a read of this and take note of its info when you design your system.