Recently I have been reading an article published by partners of internet.org. In the article its been mentioned that using PHP (a dynamic typed language) was very productive for Facebook. I tried understanding why is using dynamic typed languages productive and went through some readings. I am sharing a few which were a good read.
You can find many other resources but the above ones should give a quick overview of typing. Wiki articles are also a good place to start learning.
PS : I would strongly recommend people to read the article that is published from internet.org.