Try this:
$words = explode(" ", $_POST['input'] );
That will create an array named $words and in that it will have all of the words seperatly.
Then you would probaly have to do:
foreach($words as $theword){
/*
* You would put the search thing here...and whenever you need
* the word they put in just do: echo($theword);
*/
}
Each word will have to be searched seperatly when you split it up...but if you just put the search inside of the foreach function then it will do whatever you put, for every word.
I don't know how you have your forms setup so I can't help anymore than that...if you post the rest of your code maybe I can help more.