Output filtering does unexpected character conversion

This quick and dirty fix will not show any nodes with '<?php' in it and will format the node according to the filters you've set.

Replace:

    if($teaser) {
      $items[] = '<div id="similar-title">'. l($node->title, 'node/'.$node->nid, NULL, NULL, NULL, NULL, true) .'</div><div id="simlar-teaser">'. $node->teaser .'</div>';
    } else {
      $items[] = l($node->title, 'node/'.$node->nid);
    }

With:

    if($teaser) {
      // Norri: Skip PHP stuff, and format the other stuff better
      if (preg_match("/<?php/", $node->teaser)) continue;
      $node->teaser = check_markup($node->teaser);

      $items[] = '<div id="similar-title">'. l($node->title, 'node/'.$node->nid, NULL, NULL, NULL, NULL, true) .'</div><div id="simlar-teaser">'. $node->teaser .'</div>';
    } else {
      $items[] = l($node->title, 'node/'.$node->nid);
    }

PS: You can replace preg_match with strpos(). It will probably run faster.