HELP !!!

ok, i am not a very good programmer, but i've used PHP+MySQL+omniHTTPd for designing my web pages. here is the problem- - i have installed everything above. it just can use "GET_VARS" or something, because i have this little script (test.php): <? echo $a; $a=123; echo $a; ?> ok, i start IE, hit "localhost", it displays 123, but if i assign to "a" something like a=adadada, it doesen't get the values, still displays 123....what i've done wrong, everything is setup as ussual, i've tryed Apache instead of omnihttpd, still nothing...... please help me on this one....! :(