How to create long tail url in CI 3.

This is simply.
controller/function/var/var/var

Example:
in controller index function call var / var / var

index($var1,$var2=null,$var3=null,$var4=null){
// your code
// $var1 is bank name
// $var2 is city name
// $var3 is another name
// $var4 is another name

}

this url like this
/$var1/$var2/$var3/$var4