I think that it really depends.
If you have a programming background, everything is easier.
But then again it also depends, since you could have acquired some habits which are bad from a PHP perspective.
If you're coming from assembly language, you're not much better off since you're going to have to learn new programming paradigms altogether.
On the other hand, coming from C or C++, with knowledge in HTML, would be easy.
The syntax would be similiar, though you might face hiccups with weak typing, and pointers and references, and PHP's scoping rules.
Still, you'll just need the PHP manual, and perhaps some code examples since web development is different.
The code examples could easily be found by reading the source code of Open Source projects that use PHP.
If you were like me, and took the HTML and Javascript route to PHP, it also would be quite easy.
But trying to tackle the source code of existing projects wouldnt help much.
They would simply be too complex to handle, as the code would be structured across several (or should I say many) files, unlike normal Javascript.
In such cases, a good introductory book would be the best.
From there, you could try your own mini-projects, and gain a better grasp of the PHP manual.
There is no 'right way'. All a book does is show you the way the author would tackle the problems. If you approach the code from a different direction, and build a working solution, that is not wrong.
While I do agree with that, it is also true that if you dont have any ideas on how to start, getting to know someone else's ideas can help you jumpstart the process.