variables to deciding my Web framework for my web applications

what are different variables/concepts/frameworks which play a role in deciding building of my web applications and what are these parameters/variables choice dependent on?

Sorry , to ask this question, as its being asked at lot of places. I was not able to find a clean and clear understanding.

Each search i was able to find tutorials and guidelines for building applications using specific web framework but no where i was able to find what force drives them ?

I have gone through Ruby on Rails, Django , PHP tutorials but not able to decide which one to choose?