Getting “Cannot modify header information - headers already sent” error, but only on backend

Question

When I enable WP_DEBUG, I am getting the error:

WARNING: wp-admin/admin-header.php:9 - Cannot modify header information - headers already sent by (output started at /Applications/MAMP/htdocs/site-url.com/wp-includes/functions.php:3886) require_once('wp-admin/admin-header.php'), header

but it is only happening on wp-admin pages, not on the front end.

I have read about this in support pages like this Codex FAQ page and rather than fiddling with text editors and encoding, I simply downloaded WordPress core from the release archive for the same version that's on my site (version 4.5.5 - I am in the process of debugging before I upgrade), and I replaced my local site's version of functions.php with the functions.php file I freshly downloaded from the release archive. That had no effect.

Line 3886 of functions.php is the 3rd line below (the first if ( ! is_null( $message ) )):

if ( WP_DEBUG && apply_filters( 'deprecated_argument_trigger_error', true ) ) {
        if ( function_exists( '__' ) ) {
            if ( ! is_null( $message ) )
                trigger_error( sprintf( __('%1$s was called with an argument that is <strong>deprecated</strong> since version %2$s! %3$s'), $function, $version, $message ) );
            else
                trigger_error( sprintf( __('%1$s was called with an argument that is <strong>deprecated</strong> since version %2$s with no alternative available.'), $function, $version ) );
        } else {
            if ( ! is_null( $message ) )
                trigger_error( sprintf( '%1$s was called with an argument that is <strong>deprecated</strong> since version %2$s! %3$s', $function, $version, $message ) );
            else
                trigger_error( sprintf( '%1$s was called with an argument that is <strong>deprecated</strong> since version %2$s with no alternative available.', $function, $version ) );
        }
    }

How do I resolve / further isolate this error?

Answer 1

There's only 2 general places the problem could be: One is the theme and two is the plugins. The actual problem is that there's a deprecated function that's being called.

The header already sent message is just WordPress's way of telling you the backend broke before it could build the frontend.

I suggest you:

1. disable all your plugins;
2. install + activate a native theme like Twenty Seventeen;
3. update all your plugins
4. activate each plugin one at a time until you experience the error
5. if the error hasn't reappeared update your theme and reactivate it.

When the error reappears you know which plugin/theme is at fault and it will narrow down your search parameters. Otherwise if after all these steps the error has disappeared, the problem was out-of-date software.