For all the posts that will eventually come, PHP is a viable language to learn. I'm not saying it's the best language, nor a language that I would point to as an example of "this is how all programming should be done", but there's one big thing PHP has going for it:

For The Web, It Gets Shit Done

So, really, stop with the hate.

As for learning PHP, don't start with a framework. Learn core PHP first and then pick a framework. You'll have a better understanding about what is going on.

Join a local user group. Even if you can't make it to meetings, most of them have newsgroups that you can post questions to and get local advice. Check out http://www.zend.com/en/community/local-php-groups to see if there is one near you.

If you're not on Twitter, get on there to follow big names in the community. @calevans, @lornajane, @weierophinney, and @grmpyprogrammer are a few to follow.

Resources for help that aid in learning PHP:

• The PHP Website: http://php.net/docs.php
  
• PHP Developer: http://www.phpdeveloper.org/
  
• PHP Objects, Patterns, and Practice: http://www.amazon.com/Objects-Patterns-Practice-Experts-Source/dp/143022925X/ref=sr_1_1?ie=UTF8&qid=1330829587&sr=8-1
  
• Real-World Solutions for Developing High-Quality PHP Frameworks and Applications: http://www.amazon.com/Real-World-Developing-High-Quality-Frameworks-Applications/dp/0470872497/ref=sr_1_1?s=books&ie=UTF8&qid=1330829770&sr=1-1
  
  [Edit]
  And before I get downvoted to hell for daring to support PHP, keep in mind that I'm not saying the OP should only learn PHP. Ruby, Python, and .NET all have great things going for them, and PHP should just be one tool in a programmer's toolbox if they are programming for the web.

I'm not sure the book you are looking for exists. I would recommend a couple of different sources and, put together, would bring you to the "forefront" of PHP (this won't happen overnight tho):

• Read PHP Object-Oriented Solutions to figure out how PHP's object model works
  
  
• (Already mentioned) Check out PHP: The Right Way to get up to speed on all the latest best practices
  
  
• (Already mentioned) Learn the Symfony2 framework and the Composer dependency manager developed by Fabien Potencier. This will not only introduce you to the best engineered tools in the community but show you what a well engineered project utilizing all of PHP's best practices looks like.
  
  
• Check out Sebastian Bergmanns's Real-World Solutions for Developing High-Quality PHP Frameworks and Applications. This will give you insight into the best practices in managing a high-quality, modern PHP project.

What you have to realize is that framework is a tool for development. It lets you code pages faster, by already have done the "mindless tasks" (like creating user authentication or routing the requests) and letting you focus on the parts that matter.

But there is a cost - performance. Unless you really suck at writing code, framework will make you application slower, because of all the generalization and "works for everyone" approach.

If you have an existing site, just slapping on a framework will gain you nothing. If you want to rewrite you application from scratch, then framework will let you complete that in less time (if you already know how to use said framework).

If instead you want to expand or optimize the existing functionality, you should focus on refactoring and profiling your existing codebase. If that is the case, here are few books which would help you with it: Clean Code: A Handbook of Agile Software Craftsmanship by Robert C. Martin , Real-World Solutions for Developing High-Quality PHP Frameworks and Applications by Sebastian Bergmann and Refactoring: Improving the Design of Existing Code by Martin Fowler. Said book will not give you clear rules for "how to do this", but instead will explain you the direction in which you should aim.

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P.S. in my personal opinion , CodeIgniter is one of the worst php frameworks out there, with unreasonably high popularity.

Others have already posted replies on how to learn good PHP practices, so I won't go into that.

I find a good IDE is invaluable, and I highly recommend you pick up a copy of PhpStorm (my personal favorite) or another modern IDE - this can be extremely helpful in a number of ways.

For one, you can generate a diagram, which is always a good way to ramp up and get a high-level overview of a strange new OO codebase.

Inline documentation for property-references and method-calls is extremely useful. Being able to control-click on class-names, property-references and method-names is extremely useful too, as it enables you to follow a call-chain around the codebase quickly, and learn the keyboard shortcut for "back" so you can backtrack from where you came while following references around.

Also check out this very recent book that teaches many modern techniques:

http://www.amazon.com/gp/product/0470872497