Remembering, again, that I'm not a PHP person:
Sneaky how you can use $prizenum from 0 to 4 to pick out the prize and then use it from 1 to 5 as the placement number.
$prizes = array( 50, 25, 15, 10, 5 ); // 5 prizes, in decreasing values of your choice
$get_name = mysql_query("SELECT `username`, `offerscompleted` from `members` "
. " WHERE `offerscompleted` > 1 ORDER by offerscompleted desc LIMIT 0,5");
$prizenum = 0;
while( $namerow = mysql_fetch_array($get_name) )
$offcomplete = $namerow["offerscompleted"];
$username123 = $namerow["username"];
$prize = $prizes[$prizenum];
Incidentally, this is 100% a PHP question. Has nothing to do with MySQL, so really in the wrong forum. Oh, well.