As you can read at HowTo: Start Kirby development on a PC I use as my DEV system:
- XAMPP Version 1.8.2-5
- Apache 2.4.9
- PHP 5.4.27 (VC9 X86 32bit thread safe) + PEAR
For details look at Example #2 PHP Opening and Closing Tags, the first Note!
But keep in mind that Kirby runs on PHP 5.3 (Requirements).
You use it often within of your php files, not at the beginning.