form on first frame is submitted to page using $POST (or get if you want, but you have to change $POST below).
In the page (usually called a frameset)'s source, you have code to include the frames.
for frame b you could put
src="frameb?searchword=<?echo $_POST[searchword];?>"
(as long as the frameset is parsed by php)
And then on frame b use $GET[] instead of $POST