I'm an intermediate PHP developer with no experience building a large scale web application in this language (though I have in others, mainly Rails)...say I wanted to build a social networking site using PHP and MYSQL (preferably) with all the web 2.0 trimmings.
Where should I start? What sort of frameworks should I be looking at? Any up to date modern books that would outline something like this? Really anything for building a modern web app in PHP.
Wikipedia has a comparison of various social network frameworks / software:
http://en.wikipedia.org/wiki/Comparison_of_social_networking_software
Several of them use PHP / MySQL.
I also saw this book on creating a social network in PHP:
Create a powerful and dynamic Social Networking website in PHP
I think the place to start is to really understand the fundamentals of HTTP and the tools PHP gives you for dealing with its stateless nature.
Beyond that, I would look into templating. Perhaps Smarty?
Finally, all of the normal MVC design patterns stuff apply equally to PHP, and there are lots of implementations if you'd like to use something off the shelf. You might want to check out Cake, if you need a framework.
What sort of frameworks should I be looking at?
Try a modern framework like Kohana or maybe something more engineered-OO like Zend Framework.
You also might want to consider a simple procedural framework instead of an OO framework. PHP does very, very well working at low levels.
Avoid Cake. It tries to be Rails-like, but not only do Rails idioms translate very poorly into PHP, Cake is stuck in the design era of PHP4.
Ryan, there is a php framework called Elgg which is a php framework directed at social networking based applications.
People Pods may be what you're looking for. It's a PHP framework built with social networking in mind.
I suggest you have a look at the Yii Framework. It is very well-designed and was written with performance in mind. They've heavily focused on optimising their code for use in combination with an opcode cache like APC - no other framework has shown the same level of performance improvement when used with APC. Outside of performance, the framework also offers lots of built-in support for security (secure sessions with HMAC, SQL injection prevention, XSS prevention, etc.), forms, user input validation, caching, authentication/access control, and JQuery integration.
If you're an intermediate PHP programmer who is not experienced/confident enough to build your own framework, then Yii is a really good place to start as the code is very elegant and imho the programmer made some great design choices while writing the framework - simply reading through the Yii code makes for a great way to learn about how to design/write good PHP code.
Just my two cents...