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I've always found it frustrating in WordPress that images, files, links, etc. are inserted into WordPress with an absolute URL instead of relative URL. A relative url is much more convenient for switching domain names, changing between http and https etc. Today I discovered that if you define WP_CONTENT_URL with a relative url then when you insert files into posts they use the relative url for the src instead of absolute url. Just what I've always wanted! But the official WordPress documentation says that you should use a full URI if you are defining WP_CONTENT_URL.

WordPress codex says:

Set WP_CONTENT_URL to the full URI of this directory (no trailing slash), e.g.

define( 'WP_CONTENT_URL', 'http://example/blog/wp-content');

Everything seems to work fine when I use a relative URL, e.g.

define( 'WP_CONTENT_URL', '/my-content-folder');

But is there some problem with using a relative URI? I'm just thinking that there must be a reason for WordPress stating that it should be defined with a full URI.

Answers

I think this is the kind of question only a core developer could/should answer. I've researched and found the core ticket #17048: URLs delivered to the browser should be root-relative. Where we can find the reasons explained by Andrew Nacin, lead core developer. He also links to this [wp-hackers] thread. On both those links, these are the key quotes on why WP doesn't use relative URLs:

Core ticket:

• Root-relative URLs aren't really proper. /path/ might not be WordPress, it might be outside of the install. So really it's not much different than an absolute URL.

• Any relative URLs also make it significantly more difficult to perform transformations when the install is moved. The find-replace is going to be necessary in most situations, and having an absolute URL is ironically more portable for those reasons.

• absolute URLs are needed in numerous other places. Needing to add these in conditionally will add to processing, as well as introduce potential bugs (and incompatibilities with plugins).

[wp-hackers] thread

• Relative to what, I'm not sure, as WordPress is often in a subdirectory, which means we'll always need to process the content to then add in the rest of the path. This introduces overhead.

• Keep in mind that there are two types of relative URLs, with and without the leading slash. Both have caveats that make this impossible to properly implement.

• WordPress should (and does) store absolute URLs. This requires no pre-processing of content, no overhead, no ambiguity. If you need to relocate, it is a global find-replace in the database.

And, on a personal note, more than once I've found theme and plugins bad coded that simply break when WP_CONTENT_URL is defined.
They don't know this can be set and assume that this is true: WP.URL/wp-content/WhatEver, and it's not always the case. And something will break along the way.

The plugin Relative URLs (linked in edse's Answer), applies the function wp_make_link_relative in a series of filters in the action hook template_redirect. It's quite a simple code and seems a nice option.