I'm using Zend_Search_Lucene for my search engine. Sadly it is missing an implementation of the MorelikeThis methods which can find similar documents in the index.
Has anybody come across a decent Zend port of this function?
I found a drupal module but have no idea if this can be used with Zend without some serious hacking.
The ZF guys did a great job with Zend_Search_Lucene, but it just isn't as robust as Sphinx or Solr. I would highly recommend using either one of those, as they would give you the MoreLikeThis functionality that you are looking for, in addition to so many more features.
If you Google 'solr vs sphinx' you'll find several articles comparing the two.
Both search engines have PHP interfaces, so that makes integration easy. https://www.php.net/manual/en/refs.search.php