Personally I didn't go down the book route. A long time ago there were some great sites such as Codewalkers. Tutorials were designed to ease you in. These days most of them assume you already know PHP.
One thing I would make sure is that if you do get a book - you get one released in the last 12 months. PHP is changing massively right now, and the last 12 months has seen the biggest improvements in years, with things like the PSR coding standards, Composer packages, and finally the sense that all PHP code should be object orientated.
A lot of older books really teach PHP the wrong way and basically run through saying 'this is an if statement' 'this is a function' etc. They dont show any real practical, big usage. Most that are 3 or 4 years old dont really even go into things like prepared statements, which are an essential starting point to prevent things like XSS attacks on your database.
There's a lot to learn - only attempt it if you're seriously wanting to learn PHP. The best way to do it these days would probably be to dive right in and try creating a basic project such as a blog or cms - this can be done relatively quickly and you can re-itterate over your code until you get the hang of things, and understand how everthing works. It'll also give you a good starting point as you'd have created a 'CRUD' system (Create, Read, Update, Delete).