Why does save_post action fire when creating a new post?

When you click ‘New Post’, you’re simply loading the page wp-admin/post-new.php.

In doing so, WordPress will always create a new post (an ‘Auto Draft’) to ensure all other features (such as media uploads) and plugins work as normal, even before you actually save a draft or publish the post.

And this, in turn, triggers save_post. Hence your echo.

Okay, so why don’t I get an echo when updating or publishing?

In between saving and the following page load, WordPress is actually sending a GET redirect back to the same page, which appears transparent (you can witness this with an HTTP monitor, such as HttpFox).

In other words;

  1. You click Update or Publish
  2. Browser sends data to server
  3. WordPress handles it, and in the process triggers save_post
  4. WordPress sends back a redirection header, and exits, before any browser output occurs (including your echo)*
  5. Browser follows redirection and loads ‘edit post’ page.

The redirect might seem unnecessary (since you could just POST to the same page), but it’s part of a technique known as Post/Redirect/Get in avoiding duplicate form submissions.

If you’re attempting to print out custom messages based on a result of a function hooked to save_post, check out these questions/answers.

*Not strictly true, your echo will in fact occur before the redirect header is sent, but the browser will either discard it, or things happen so quickly it never renders.

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