web crawling PHP?

Hello,
I am supposed to construct a page that searches in specific websites to extract information, like those sites from where you can rent a car for example. There is a form in the site where the user selects some fields (for instance departure and drop-off date), then the data are submitted to the other page that searches 2-3 sites and finds which cars are available on those dates.
I wanted to ask if there are ready scripts to do that, if not, some hints on how to start.
I am familiar with PHP forms and data extraction from mysql databases, but when you extract data from other sites, I have no clue how I can begin and deal with it...

The problem with what you are trying to do is that sites that work this way, eg., hotel bookings, flights, etc, have agreed to make their data available at a central point, to which users subscribe, and you pick it up from there.

It would be very difficult if not impossible to just pick some sites at random and hope to extract the kind of data you are looking for.

This sounds like a uni project?

well no, it will be a project for me as freelance, and wanting to get some hints on that before I accept it. So, if you have a site that offers info on car rentals (from companies), the site you have will probably have some more info on how to search in the companies' sites, no? I mean the companies will grant access to our scripts so that we will check availability, prices etc...

I think you need to come at this from a different angle.

First task would be to pick the car rental sites that you would like to link up with and ask them if they will make their data available to you.

If you don't get any good responses, then the job is a non-starter.