<?php

$str = "Ololo   lalala     test  lorem ipsum.";

function repl ( $str, $spaces = "", $down = FALSE) {
  return $down ?
    (strlen($spaces) > 0 ? repl (str_replace ( $spaces, "_", $str ), substr($spaces, 1), $down) : $str)
  : (5<=(strlen($spaces)) ? repl($str, $spaces, TRUE) : repl($str, $spaces . ' '));
}

echo(repl($str));

?>