I want to write an app for facebook. Googling suggest me that the best language and technology will PHP. But the thing is I have never develop anything on PHP.
My Question is which PHP version I should. Which IDE, where to start web development and where to start facebook API
Which IDE: I prefer Netbeans; Eclipse is alright too.
Where to start: http://developers.facebook.com/docs/
You should get a LAMP/WAMP package and set up a server on your machine to play around with PHP first. I personally use the Uniform Server; it's lightweight and requires nearly no set up.
Start with php 5.3. You should either get a book on PHP, or read the online documentation at http://php.net/
If you already know how to program in some language, then PHP's not too hard. You can get documentation for any function using http://php.net/function-name
As far as IDEs go, use whatever you're comfortable with. PHP doesn't care. I personally just use gvim to write my code and view it using apache+firefox.
which PHP version
This is usually dictated by whatever PHP version your host provides - if you're shopping around, or have direct choice, go for the latest version. If you want to build a portable app, you may need to look into supporting multiple versions (4 and 5) although 4 is fairly out-dated at this stage - many apps have dropped support.
Short answer: 5.3
IDE is entirely personal - what do you currently use? I don't personally use one - just a text-editor and a local (XAMPP) server.
I recommend you to start with basics.
Install the XAMPP webserver package on your local machine. Its easy and you don't need to config php, apache etc. Its everything done by default.
http://www.apachefriends.org/de/xampp.html
Then I would try first tutorials to write functions, classes, etc. Handle with variables.
Introduction: http://tut.php-quake.net/en/ Reference: http://php.net/
As an IDE there is just eclipse (for me). But if you never had worked with eclipse, it could be an overflow for you. Too many things yo don't need in the beginning.
http://www.eclipse.org/downloads/packages/eclipse-php-developers/heliosr
It has a good code completion and syntax highlighting.
Afterwards you can begin with Facebook Samples. Best thing: Download, modify, try, again.
http://developers.facebook.com/docs/guides/web
Good Luck!
I really want to point, that you can get lots and lots of information about PHP and it's functions on official site - http://php.net/
This is the number one place for me if I cannot get something to work or need to see syntax for some rarely used functions. And yeah, all my PHP knowledge comes from php.net function references and manuals, so... I'll say: consider buying a book only then if php.net gave you nothing, what I doubt.
Oh, yeah, forgot before... I've heard lots of good feedback from beginners after they've read http://devzone.zend.com/article/627 (PHP For the Absolute Beginner) !