$kod = $_POST['kod'];

if ( $kod == 'a' ){
<META HTTP-EQUIV="Refresh" CONTENT="0; URL=a.html">
}elseif ( $kod == 'b'){
<META HTTP-EQUIV="Refresh" CONTENT="0; URL=b.html">
}else{
<META HTTP-EQUIV="Refresh" CONTENT="0; URL=c.html">
}