How can I insert PHP code in Views? I tried the views_php but I have no luck. Any link to its documentation? 

I want to add some custom field with PHP code inside it but the one I did return nothing. Please see the attached picture.
Yes you can add php code using views_php

Add field in views global

Then add code you need is in Output code sector. I thing here is the problem for you. you add php code to 'Value code'. 

Then look at the preview. it's done!!!!!!