How to use PHP Evalution in rules scheduling
After struggling so many days I found the solution that how we can do scheduling using php evaluation. To do this I have simply pick the value in data selector(under my rule) that is set by user in his rental form as shown in my 1st screen short. Now I need to schedule that rental date according to the alerts set by user in alert section (screen short2) where i was stuck.Under PHP evaluation section we have to use $value variable that returns the value that we selected in data selector.In case of dates  it returns the timestamp not the actual date. So then I used PHP code to add/substract the days according to the user set in alert section and succeed to do scheduling.   

I hope it will helpful for others.