if its available, it will be in the $_SERVER['HTTP_REFERER'] it wont always available.
$referer= $_SERVER['HTTP_REFERER'];
//find the search query from google that brought them here
$qref= strpos($referer,'google');
if($qref!=”){
$qstart = strpos($referer,'q=');
$qend = strpos($referer,'&',$qstart);
$qtext= substr($referer,$qstart+2,$qend-$qstart-2);
$qtext= str_replace('+',' ',$qtext);
}
echo $qtext
above untested grabbed from another site.