Intellisense in Vendor directory

I have a project in vendor directory that uses functions that  have been defined in the main project, but PHPStorm marks those as Undefined. 

For instance, in the file in vendor directory I have a function that looks like this:

function do_something_with_input($input) {
  return absint($input);
}

And, PHPStorm marks `absint` as Undefined, and offers to create it. That function is defined in the main project already. When I move this code outside of `vendor` directory, PHPStorm is not marking `absint` as Undefined. 

How can I have PHPStorm properly apply Intellisense to code in vender directory by using te whole project? 

Milan