How is it better than Netbeans or Eclipse?
It's been a couple of years since I last looked at Netbeans/Eclipse, so take this with a grain of salt: PHPStorm just works.
The biggest issues I used to have were finding Netbeans very slow, and auto-complete in both Netbeans and Eclipse being somewhat unreliable - great when they worked, but that wasn't 100% of the time.
Essentially, PHPStorm is what I always wanted Netbeans/Eclipse to be.
As I say, though, it's been a long time since I last looked, and that's for a very good reason: at long last, I'm completely satisfied with my IDE.