i want to modify search.php to add a line ,and whit that line i want to search to another site. How i can do this.

Nobody can do that ?

you have to edit include/searchform.php or search.php

You might want to rephrase your question with more details so that we can provide a cogent reply. As mjoel said, the search feature is in the files (from x.root) search.php and include/searchform.php. However, it sounds like you're trying to figure out how to include search results of another site in your search page. That might not be so simple. It won't be just a one-line solution. However, there is a module called SnX MetaSEarch that searches several search engines at once and returns results that might help, and there is an RSS module called RSS Center which might allow you to grab the other site's contents and make them searchable on your site. There is also Happy Search module which searches your site and Google. And XoopsAddOns has a Google Ajax Search embedded in their search pages, so they may have some thoughts on how to do this as well.

Good luck, and share the results once you figure it out.