<?php

$text = 'll the Lorem Ipsum ##generators## on the Internet tend to repeat predefined chunks as necessary, making this the first true generator on the Internet. It uses a dictionary of over ##200 Latin words##, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always ##free from repetition##, injected humour, or non-characteristic words etc.';

preg_match_all('/##[^#]+##/', $text, $matches, PREG_SET_ORDER);

for ($i = 0; $i < count($matches); $i++) {
  $text = preg_replace("/".$matches[$i][0]."/", "___".strval($i+1)."___" , $text, 1);
}

echo $text;

?>