You don't. An iframe is a client controlled markup tag which allows you to import a document from an alternate location.
If you want to search something using PHP, you'll need to read the contents of the source, and search from there. You can read the content using file_get_contents if you have allow_url_fopen directive enabled, or you can use curl or sockets to load an external page.
header('HTTP/1.1 420 Enhance Your Calm');