adding to existing uneditable site

[phpstriker]

I'd like some pointers or at least some help as to what to search for.  As an example, I want to use php to allows those visiting my site to browse realtor.com, but add some things to the pages it returns.  For instance, if i detect that a user has come across a certain house, i want to add a link to another site, and make it look like it was a part of the realtor.com page.  I also might want to add some css so that whenever a page being visited meets certain criteria, I display certain css to highlight an area of the page.

 

Is this possible? How do I do it? What terms do I need to search for examples?

 

Thanks!