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...