Similar Posts


Unfortunately, due to ill-health, this plugin has not been developed or supported properly for some years. It works with the latest versions of WordPress (including on this website) but could possibly conflict with any WordPress features added after 2008 — e.g. custom post types — if you use them.


This plugin displays a list of posts which are related or similar to the current post.

This is version download latest version. It is compatible with WordPress 1.5–2.6.2.

  • fixes a problem with stemming and stop words and offers a new fuzzy matching capability; supplies a .pot file making internationalisation possible; introduces the {imagealt} output tag and allows {excerpt} to output whole sentences; the content filter and the widget can now take a parameter string; output can be automatically placed after post content without editing theme files.
  • fixes the German-language stemmer which should have been encoded as utf8.
  • fixes the Italian-language stemmer which was crashing under PHP4.
  • allows the current post to be marked manually where the automatic mechanism fails; when used as a widget the plugin now honours the setting to show nothing when there is no output; {commenterlink} now applies the appropriate WordPress filter; and fixed a problem with some installations not finding the right language files.
  • fixes the option to include attachments and adds a parameter to the {imagesrc} output tag to append a suffix to the image name.
  • has a new option to include posts which are attachments; a new output template tag {authorurl} which points to the archive of the author’s posts; new behaviour for the {php} output tag which can now accept other output tags in the code; and includes a fix for MySQL problems in some locales’.
  • provides the ability to select from two algorithms for term extraction; allows you to specify post relationships by hand; and fixes a problem indexing tags in some languages.
  • adds an option to match the current post’s author and extends the options for snippet and excerpt output tags to make the ‘more’ text into a link.
  • adds an option to show posts by status, i.e., published/private/draft/future, changes the {categorynames} and {categorylinks} output tags by applying the ‘single_cat_name’ filter, and fixes a bug in WordPress pre-2.2 that stopped installation code running on Windows servers.
  • 2.5.0 improves the CJK matching algorithm by using bigrams. Also introduces a new output tag {imagesrc}, and adds more parameters to {image}. Fixes bugs with empty categories, excluded posts, and the option to omit current posts.
  • 2.5b28 improves the matching algorithm and adds an experimental mode for blogs in Chinese, Korean, or Japanese.
  • 2.5b27 fixes a bug with the bulk indexing of tags.
  • 2.5b25 makes some important changes: the {image} output tag now serves real thumbnails (couple of bug fixes too); output can now be sorted as you choose with sub-headings included; the {date:raw} tag modifier has been added to help the sorting; the ‘trim_before’ option has been replaced with the more logical ‘divider’.
  • 2.5b24 fixes to stop recursive replacement by content filter, {gravatar} to allow for ‘identicon’ etc., to {commenter} to allow trimming, and to remove a warning in safe mode
  • 2.5b23 brings a new option to filter on custom fields and adds proper nesting of braces in {if}.
  • 2.5b22 moves the manage menu under settings as a subpage, restores automatic indexing on activation, fixes conflicts with the legacy Similar Posts Feed plugin, fixes bugs in several output tags, and introduces the option to show only pages.
  • 2.5b20 doubles the speed of indexing and reduces the memory footprint considerably.
  • 2.5b19 fixes a bug when snippets are stripped of extra tags.
  • 2.5b18 fixes a problem with filtering the output and introduces the conditional tag {if:condition:yes:no}.
  • 2.5b16 fixes a problem with {php}.
  • 2.5b15 fixes for some more installation problems, one or two bugs, and adds the ‘included posts’ setting.
  • 2.5b14 fixes for some of the kinds of installation problems.
  • 2.5b11 fixes some widget problems.
  • 2.5b10 fixes (some?) of the problems folks have been having with no posts found. Most of such errors seem to arise when the proper table is not created and this version addresses that.
  • 2.5b9 has new features and improvements.
  • 2.3.6 restores the widgetiness I managed to remove in 2.3.5!
  • 2.3.5 has been rebuilt to save memory and can match the current post’s tags. It also fixes a bug with categories in WordPress < 2.3.
  • 2.3.4 now works as a widget.
  • 2.3.3 beta adds the ability to include as well as exclude categories and authors and is able to find posts by tag.
  • 2.3.2 beta fixes a conflict between tags and categories.
  • 2.3.1 beta fixes a stupid bug in category exclusion.
  • 2.3.0 beta is compatible with WP 2.3, fixes the {author} bug, and a number of problems related to versions of MySQL.
  • 2.1.1 beta fixes a badly chosen fallback value for the number of terms used to match similar posts.

Ideally, similarity or relatedness would be based on a post’s meaning. Tagging systems try to add meaning after the fact but suffer from two deficiencies, one practical and the other theoretical. When a blog already has many posts it can be impractical to retrofit a tagging system by tagging every post by hand. ‘Automatic’ services, like Yahoo’s, tend to produce too many suggestions which need to be culled, again by hand.

The theoretical problem with tagging is that it tries to pin down a meaning for a post by categorising it under a small number of types, whether those types belong to a predetermined hierarchy or arise by ‘folk’ classification. In fact, a post has a variety of meanings, a multitude of ways it can be related to other posts. Meaning doesn’t just lie in the intention of the author or in the classification of the reader; meaning also inhabits the text itself. Meaning is in the words.

The Similar Posts plugin compares posts by comparing their words. MySQL has a sophisticated full-text searching facility with a carefully tuned algorithm for judging the similarity between texts. Similar Posts extracts representative words from a post’s content, title, and tags and uses the full-text index to find the best matches between posts. This simple approach gives surprisingly good results.

The results can be tweaked in several ways to tailor them for you blog. By default the plugin chooses the 20 most frequent words to make its matches but the number is adjustable. It is worth experimenting to see how many words gives the best results for your blog — it has hardly any impact on speed, even if you set the value high enough to include the whole post. The relative importance given to words in your title may be adjusted so that well-chosen titles can be used to advantage or titles with little relevance downplayed. Similarly, tags can be used to improve matching or not according to your blog and it’s needs.

It is also possible to override the automatic similarity ranking by using a custom field. In the post edit screen create a custom field called ‘sp_similar’ with the ID value of the post to which you wish to ‘link’. You can link to multiple posts by entering a comma-delimited list of IDs.

The plugin has a settings page which lets you change how the output is generated and displayed. There is also a management page where you can change settings which affect the index.

Note: Similar Posts needs to know the ID of the post for which it is generating related posts. WordPress keeps track of that information in a global variable but unfortunately some other plugins can corrupt the data before Similar Posts gets a chance to use it. Similar Posts tries various tricks to get round this but sometimes it fails. The usual symptom is a list of similar posts that stays the same from page to page. You can help Similar Posts out by marking the current post manually by adding a line to your theme files. Find the place where the_content(); is used to display the current post and right after it put similar_posts_mark_current();.

Installation Instructions

  1. If upgrading from a previous version, first deactivate the plugin via the Plugins page and delete the plugin folder from your server.
  2. If you have been using the Similar Posts Feed plugin you should deactivate it as it is now obsolete.
  3. Upload the plugin folder to your /wp-content/plugins/ directory. You will also need to install the Post-Plugin Library.
  4. Go to your admin Plugins page and activate Similar Posts. This will automatically add a new table to enable fast, flexible full-text matching. If the plugin reports that there was a problem creating the table first try deactivating and reactivating the plugin.
  5. Put<!--?php similar_posts(); ?--> at the place in your theme files where you want the list of similar posts to appear. Lorelle on WordPress has a good guide to modifying themes for plugins.If you are averse to editing template files you can also place the post listing automatically either as a widget in the sidebar of your widget-aware theme or after each post (from the plugin’s Placement submenu).
  6. Use the admin Settings|Similar Posts pages to set all the available options. Alternatively, the options can be overridden by passing a parameter to the similar_posts template tag.

Usage and Options

The configuration page will help you to set up the plugin to your satisfaction.

The Index Management Page

Using this settings subpage you can re-index your blog. There are two main settings which affect the indexing.

PHP is, by default, not very good at handling text that isn’t in English and you might find Similar Posts mangles extended characters. If so, you can get the plugin to use PHP multi-byte string library if it is available.

The second setting attempts to handle words with related meanings. For example, ‘animal’ and ‘animals’ should probably not count as two distinct words, nor ‘follow’, ‘follows’, ‘following’, etc. You can choose to build the index using a stemming algorithm that groups such words as one (if there is one available for your language) or you can try the fuzzy matching algorithm. Whether it is better to be strict or to be relaxed will depend on your website.

A third setting is for blogs written mainly in Chinese, Korean, or Japanese. The MySQL fulltext index used by Similar Posts has problems with these languages but this setting tries several ways to work around the issues. The setting currently only works when posts are encoded as UTF-8. I would be very glad to get opinions from users familiar with these languages.

To avoid excessive memory use the indexing routine processes posts in batches of 100. This figure can be reduced to shrink the memory consumption even further.

Language Issues

The underlying MySQL full-text indexing is obviously very locale-dependent — how words are divided or punctuation handled, what words are treated as noise, etc. all vary from language to language. For the Similar Posts plugin to work well the version of MySQL on your server must be properly setup in the appropriate language.

Similar Posts generates the terms it matches on by analysing the word frequency of a post while ignoring the most common ‘noise’ words — in English, words like ‘of’, ‘and’, ‘across’, ‘someone’, etc. It uses a so-called ‘stop list’ of common English words to ignore. In fact it uses the stop list a standard English installation of MySQL uses. Obviously this list will be useless for other languages so Similar Posts makes the stop list pluggable.

The Similar_Posts folder contains a subfolder, ‘languages’, with stop lists and stemmers for a German, English, French, Spanish, and Italian. The plugin checks the WPLANG constant (defined in wp-config.php) to see which language WordPress is using and looks for a file on that basis. If WPLANG is undefined or the appropriate file cannot be found the default English list is used.

If you are looking for help setting up a stop list in a language other than English a good resource can be found at Stemmers in PHP are harder to come by. You can work out how to adapt any you find by inspecting the provided stemming files

414 replies on “Similar Posts”

  1. How about the performance of this plugin? I mean, if it check the posts’ body to find related posts, it may cause the high load on server. I need a light related post plugin, can this plugin satisfy me?

  2. Hi

    when a new post is added the similar posts displays as none found, untill i click “Recreate Index” then the similar posts are displayed.

    How can i set the “Recreate Index” to auto run every time a new post is created, or if i set up a cron job what code would i execute?

  3. >How about the performance of this plugin? I mean, if it check the posts’ body to find related posts, it may cause the high load on server. I need a light related post plugin, can this plugin satisfy me?

    I too would love to know if this wordpress plugin will cause high CPU usage.

    >Hi, do you still develop the plugins? Will it work under WordPress 2.8?

    This too — will it work right with WordPress 2.8 and are you still planning to keep it updated?

  4. I too would love to know if this wordpress plugin will cause high CPU usage.

    This too — will it work right with WordPress 2.8 and are you still planning to keep it updated?

  5. Hello I love you plugins! all of them!

    Today i found this under Filter settings >> categories to exclude/include :
    Warning: str_repeat() [function.str-repeat]: Second argument has to be greater than or equal to 0 in /customers/ on line 593
    Is ‘t something to worry about?

    Thanks Helena

  6. Rob, I was having issues with this plugin, but found my answers here on your blog! Thank you for being so active in the success of your plugin and the success of others! Your plugin is really great for keeping people on my blog and delivering even more value! Really appreciate you!

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