Something

<?php

  function returnStringRST($text, $openingMarker, $closingMarker) {
   $openingMarkerLength = strlen($openingMarker);
   $closingMarkerLength = strlen($closingMarker);

   $result = "";
   $position = 0;

   if(($position = strpos($text, $openingMarker, $position)) !== false) {

     if (($closingMarkerPosition = strpos($text, $closingMarker, $position)) !== false) {
       $replaceThis = substr($text, $position-$openingMarkerLength, $closingMarkerPosition - $closingMarkerLength);
       $text = ereg_replace($replaceThis, '', $text);
       returnStringRST($text,  "<!--", "-->");

     }
   }
   return $text;
  }


  $testText = "Before comment <!-- comment--> before comment2 <!-- comment2 --> after comment2";
  $result = returnStringRST($testText, "<!--", "-->");
  print($result);

?>