Is there an easy to understand scheme to decide what kind of code belongs to a plugin or the theme’s functions.php?

There are many cases and many debates about that topic, mostly because there are some misconceptions about the inner workings of WordPress. I am asking for an answer based on facts, not on opinions.

It should explain how to handle these points (and probably more):

There are often pros and cons for both sides. Our most popular question Best Collection of Code for your functions.php file got a lot of code snippets as answers that are at least debatable.
We need criteria a beginner can understand, maybe a check list – with reasons.

See also the related question by Chip Bennett on our meta site: Questions specifically asking for a solution "without a plugin"

Related: Where do I put the code snippets I found here or somewhere else on the web?

  • 1
    I wonder what would constitute facts for the purpose of this question. Person A says CPT go in plugin, Person B says CPT go in theme. How can we procure a fact to validate one of opinions? This might be dangerously close to "not constructive".
    – Rarst
    Commented Nov 18, 2012 at 18:24

7 Answers 7


I would start with this question: Is the functionality related to presentation of content, or with generation/management of content, or of the site, or of the user identity?

If the functionality is not related specifically to presentation of content, then it is squarely within Plugin Territory. This list is long:

  • Modifying core WP filters (wp_head content, such as canonical links, generator and other HTML meta, etc
  • Site Favicon
  • Post-content shortcodes
  • Post sharing links
  • Google Analytics (and similar) footer scripts
  • SEO tools/controls
  • etc.

If the functionality is related to presentation of content, then it is a candidate for being included in the Theme. At this point, I would revert to @Raf912's Theme-switch criterion: would you miss the functionality when you switch Themes? If the answer to that question is no, then the functionality belongs in the Theme. Some examples:

  • Removing/overriding the WP core Gallery CSS
  • Filtering post excerpt length, "read more" text, etc.
  • Anything implemented via add_theme_support() (I suppose this one should be obvious)
  • Custom CSS

Normally, these two questions will provide a fairly clear line of differentiation; however, there are exceptions.

Custom Post Types

Custom Post Types, for example, are a bit of a unique hybrid of content generation and presentation, given the way the Template Hierarchy works for single-post-type archive index pages and single post pages. The content-generation aspect of CPTs would normally place them squarely in Plugin Territory; however, Plugins cannot define template pages that inherently fit into the design/layout/style for any given Theme (especially if the CPT displays other than the usual Title/Content/Meta, or has custom taxonomies associated with it).

Long-term, the solution to this disparity, IMHO, is to have a standard convention/consensus for the definition of CPTs for given types of content (real estate listings, calendar events, e-commerce products, book/media library entries, etc.). That way, user-generated content would remain portable between Themes that implement the standard/convention definition of a given CPT, while Theme developers retain the flexibility to define the design/layout/style of that CPT in the Theme template files.

Social Media Links

Similarly, I would normally say that social media profile links, ave become all but ubiquitous in current Themes, are Plugin Territory, because they have nothing to do with presentation of content. The best solution would be for these profiles to be defined somewhere in core; however, there is currently no standard/consensus means of defining these links. Are they best-defined at the site-setting level, or on a per-user basis? If per-user, which user's meta gets exposed in the template? etc.

So again, long-term, the solution to this disparity is for either core to define where these links are defined, or else for the Theme developer community to develop its own consensus. In the meantime, there's really nothing for it but to keep them defined within each Theme.

  • add_theme_support( 'automatic-feed-links' ); is not presentational. But it is required by the theme guidelines. Why is it a necessary risk to loose this functionality after a theme switch?
    – fuxia
    Commented Nov 19, 2012 at 22:01
  • 1
    Anything that is implemented via add_theme_support() can only be implemented via the Theme. Using add_theme_support( 'automatic-feed-links' ) within the Theme actually ensures a consistent experience from Theme to Theme, since the feed links generated will be the same. Commented Nov 19, 2012 at 22:48
  • 5
    I think it is misnamed: Feed links are not presentational. If the next theme doesn’t call that function the user will lose the feed links. And you can add that per plugin without any problem. That’s why I am confused about it. :)
    – fuxia
    Commented Nov 19, 2012 at 22:55
  • 1
    You know: that's a good point. :) Commented Nov 19, 2012 at 23:49

An easy test where the code is best placed:

  • write the code into the functions.php
  • switch theme
  • do you miss the functionality, is the blog not proper working or fragments of the old theme (e.g. shortcodes) are left?

    • yes: put it into a plugin

    • no: leave it in functions.php

Examples: Write a shortcode. After switching the theme, the plain shortcodes are left in your posts. So it will be better placed in a plugin.

Write a function to list the last comments. After switching the theme, everything is ok because maybe the other theme have an equivalent function.

It really depends on the code and what it will do. Some code only influence the styling or content of the theme, some others will modifiy blog posts.

  • 12
    +1 If the code is specific to the theme, put it is functions.php. If it needs to apply to more than one theme, put it in a plugin.
    – s_ha_dum
    Commented Nov 18, 2012 at 15:15

I don't think there's an easy answer to this question, but I bet we could make a flow chart to help with the decision. Here's a rough outline of such a flow chart, which can and should be expanded. Comment with suggestions!

  • Is this code to be hosted on a single-site installation of WordPress?
    • Yes - Does the site's theme only change with major redesigns and shifts in functionality?
      • Yes - Is the code in question specific to this current design?
        • Yes: functions.php
        • No: Plugin
      • No (it changes often or at a whim) - Plugin
    • No (Multsisite) - Are you hosting the multisite installation OR is it a hosted multisite solution that allows plugins?
      • Yes: Is the functionality in question specific to this site, or can/should it be used by other sites in the network?
        • Specific to this site: functions.php
        • Shared amongst multiple sites - Do you want to force it on every site?
          • Yes: Plugin, stored in mu-plugins directory or network-activated
          • No: Is this a network of unrelated sites? (e.g. different clients)
            • Yes: Would it be bad or unprofessional if client A saw or activated the plugin you wrote for clients B, C, and D? (e.g. maybe it would break the site or cause undesirable functionality)
              • Yes: functions.php
              • No: Plugin
            • No: Probably plugin
      • No (hosted by a service like VIP that doesn't allow plugins): use functions.php
Some other thoughts that I didn't know how to fit in here:

  • Parent themes -- sometimes with shared functionality it would be better to make a parent theme and put the functionality in the parent theme's functions.php file.
  • Plugin directories of large multisite installations can quickly become unruly, so sometimes shared functionality used by a low percentage of sites (e.g. < 1%) would be best to duplicate in functions.php files.

From here Themes VS Plugins

Add custom code to a child theme so when you update the parent theme, your custom code is not lost.

You can also create a site specific plugin which contains all your custom code as well.

As far as writing code versus plugins, you can use plugins to and the functions however for most of what you want, hand coding is the best as its easier to modify except in some cases like meta boxes where you may consider using a plugin unless you're a theme developer.

 function modify_contact_methods($profile_fields) {

// Add new fields
$profile_fields['twitter'] = 'Twitter Username';
$profile_fields['facebook'] = 'Facebook URL';
$profile_fields['gplus'] = 'Google+ URL';

return $profile_fields;
add_filter('user_contactmethods', 'modify_contact_methods');


  1. Add new custom post type - Code
  2. Add new fields to the Users - Code Above
  3. Add new widgets - Code
  4. Add custom permalinks - WordPress Permalink Settings

I know this is a dead horse and that Chip has pretty much covered it, but wanted to add a few thoughts.

If you make a living programming and find yourself working on wordpress sites under deadlines, you are going to find that it really comes down to time.

More often than not, especially for those just starting out, it is much faster and simpler to just add whatever you need into a theme and call it done.

That being said, if you work on wordpress on a semi regular basis, you should seriously consider doing the following:

  1. Build out a plugin skeleton

This should handle everything that you will commonly need to do with a plugin, including activation, deactivation, version updating, building admin panels, and uninstalling.

If you make the time to do this, you will find:

  • It no longer takes much extra time to add functionality via plugins
  • You can start building a solid list of plugins to re-use on other projects as needed, saving yourself a lot of time in the long run.
  • You can make them publicly available if you want extra visibility

You can now build things out properly and get future projects done more quickly.

  1. Build out a theme skeleton

This should handle everything that is commonly needed in a theme:

  • A core style sheet containing styles that you use commonly (resets, etc.)
  • A proper index.php file, handling everything you need for any template
  • A functions.php file - you won't use it nearly as much, but it will still come in handy.

Once you have that done, build out a child theme skeleton that uses your primary theme.

  • Add the stylesheet, referencing your parent theme.
  • Add the functions.php file

Once you have these two things done, creating new sites for people becomes much faster.

If you do the above, you can then work on the following:

  • Spend your new found free time getting more familiar with PHP, WordPress, JavaScript, CSS, and/or mySQL ... the more you learn of these the faster you will get things done.
  • Update your plugin, theme, and child theme skeletons as you find things you should improve. No matter how good you are, if you keep learning you will find improvements to be made.

And, if you do all of the above, you will find that Chip's answer will then not only be ideal, it will become optimal.


The simple answer is this..

Is the code dependent on any of the functionality built into a specific theme? If yes, then put in a theme.

Do you want this code to be transferable between sites and between themes? If yes, then put in a plugin.

If the answer is no to both of the above, then picture the site 5 years in the future, when it's time for a redesign. Is the function of the code you're writing something that will survive the next design update? If yes, put in a plugin.

Also, if you're not using child themes and you plan to update the theme, I would also suggest you use a plugin.


To answer OP:

custom post types and taxonomies

Should go in a plugin as they define stuff on a structural/conceptional level, not a presentation level

contact forms

Are presentational should go in theme functions


In plugin as structural

custom widgets

Are very much presentation and should go with the theme: functions

add_theme_support( 'automatic-feed-links' );

Theme support goes in theme functions

SEO functions like custom meta elements

Presentation, thus in functions

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