So I want to create a website that allows me (or others) to type in a words and select a relevent search, and have it search multiple XML pages on another server (IE: they search my site and are given 3 other sites to go to with that relevent information).

The XML documents are setup and available on the other websites server but I want to access it through mine.

How can I do this and how can I use PHP to allow them to search through, and then display the relevent information?

You can search through XML using xpath (http://www.php.net/manual/en/function.xpath-eval.php) expressions, but if the other sites provide search results as XML, you then just need to display them.