OOP abstraction with CodeIgniter Models - php

Question

I'm writing a library search engine where a user can search based on various criteria (e.g., author, title, publisher, etc.) with CodeIgniter. So, I defined the interface BookSearch which all classes responsible for searching the database will implement
interface BookSearch{
/**
Returns all the books based on a given criteria as a query result.
*/
public function search($search_query);
}
If I want to implement a search based on authors I can write he class AuthorSearch as
class AuthorSearch implements BookSearch extends CI_Model{
function __construct(){
parent::__construct();
}
public function search($authorname){
//Implement search function here...
//Return query result which we can display via foreach
}
}
Now, I define a Controller to make use of these classes and display my results,
class Search extends CI_Controller{
/**
These constants will contain the class names of the models
which will carry out the search. Pass as $search_method.
*/
const AUTHOR = "AuthorSearch";
const TITLE = "TitleSearch";
const PUBLISHER = "PublisherSearch";
public function display($search_method, $search_query){
$this->load->model($search_method);
}
}
This is where I hit my problem. The CodeIgniter manual says that, to invoke a method in a model (i.e., search), I write $this->AuthorSearch->search($search_query). But since I have the class name of the search classes as strings, I can't really do $this->$search_method->search($search_query) right?
If this was in Java, I'd load objects into my constants. I'm aware that PHP5 has type hinting but the target platform for this project has PHP4. And also, I'm looking for a more "CodeIgniter" way of doing this abstraction. Any hints?

Answer 1

You can really do $this->$search_method->search($search_query). Also in CI you can assign library name as you want.
public function display($search_method, $search_query){
$this->load->model($search_method, 'currentSearchModel');
$this->currentSearchModel->search($search_query);
}

Answer 2

What you're talking about it the driver model. You can, in fact, do what you're suggesting can't be done:
<?php
$this->{$search_method}->search($search_query);
CodeIgniter has CI_Driver_Library & CI_Driver classes to do this (See CodeIgniter Drivers).
However, I've found that it's usually simpler to implement an interface / extend an abstract class like you're doing. The inheritance works better than CI's drivers.