Wordpress - Where to put my code: plugin or functions.php?

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.

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.

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.