<?php

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

function five($n){
	return $n >= 5;
}

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

echo(repl(five, $str));

?>