If you are asking about the operating system then the Linux is preferred.
If you are also interested to know the application which is good for PHP coding then I would suggest using Eclipse or Komodo on Linux. But if you are on MS-Windows there's no match of Adobe Dreamweaver (or any older version of Macromedia Dreamweaver).
par
Kashif Kamran , Head of Technical Success , Aurea Retail
By Platform, I assume you refer to framework for PHP development. If yes, look into CI (CodeIgniter) and Yii. I have found them pretty handy, especially in CRM Development.
PHP is famous because it is an OpenSource and free script language and it works with MySql and Linux. All of three things are free and this is another reason about its popularity.
So Linux is best platform for PHP to be run.
Operating System: Linux is best for this purpose
Server: Nginx, NodeJS are best as these are non blocking ones
Framework: depending upon the scale or type of your application. Scale wise lightweight frameworks include CodeIgniter or Yii where Yii is also increasing its popularity among large level application designers each day, For large applications you can use CakePHP, Zend, Symphony. or there are many other platforms depending upon the type of your application like blogs, CMS, Ecommerce etc
