Friendlier "headers already sent" error message on wp-login.php

[johnbillion]

When a user first installs WordPress and manually configures their wp-config.php file, a rogue leading or trailing space, tab, new line, or other character in this file can prevent the user from logging in due to the "headers already sent" error. See #36213 for a recent example.

In order to make this error condition friendlier, wp-login.php should perform a headers_sent() check before sending cookies and display a friendlier error message if headers have already been sent. Rather than a verbose error message, a link to a relevant Codex or Handbook article could be provided.

[ericlewis]

Great idea!

[juanfra]

Print error if headers were sent already

[juanfra]

Hello,

I think this would be great for newcomers. We should define a string for the error. I've suggested one in my patch.

Also, I've seen there are a lot of non-compliant code regarding the WP coding standards. Wouldn't be this a good opportunity to work a bit around this file?

We can also add some html to make the message look prettier. Should I try something out? (Something somehow closer to the look and feel of the login page).

Regards,
Juanfra.

[DrewAPicture]

Assigning to mark the good-first-bug as "claimed".

See 36215.patch​

[johnbillion]

It turns out that this functionality is already in place. See the above screenshot.

I thought about suppressing the Cannot modify header information - headers already sent errors that appear if error output is enabled, but this actually proves to be a very effective phrase to search for in addition to following the links in the above message.

[juanfra]

Good! That looks great. The only thing (from what I've tested) is that the error message is displayed when somebody goes (or is redirected) to wp-login.php directly. If you visit wp-admin/ there's not any kind of notice, and you get the php error.