Help needed Multi city flight search logic

If every option/input/dropdown etc on the search area is selected, I believe it will be the same with the "one input search" you can already do. 
Just send the values to controller, then to model and retrieve the results by filtering according to the values.

However, if there are some options which might be blank, you can check this answer written for one of my questions : 

https://forum.codeigniter.com/thread-74617.html

Good Luck.