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I'm getting an error on my site regarding "headers already sent":

Warning: Cannot modify header information - headers already sent by (output started at ........./wp-admin/menu-header.php:161) in ....../wp-includes/pluggable.php on line 881

I read the Wordpress FAQ that discusses this, but - as you can see - this error is caused by the Wordpress core (and not even at the end of a file).

Interestingly, I don't get this error on my local machine. Only on my server.

What can I do about this?

I'm running Wordpress 3.4.2

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    Something is messing up your admin menu. Maybe a plugin has registered an options page with broken parameters? Disable all plugins and switch to TwentyEleven. Does it still happen?
    – fuxia ♦
    Oct 17, 2012 at 23:59
  • @toscho - Actually, now that you're saying, I realize that I probably messed it up. I register my page under the admin_menu action hook, and then do a wp_redirect from within that page (wp_redirect is defined in pluggable.php). Is there a different (earlier) hook I should be using instead?
    – Myself
    Oct 18, 2012 at 2:26
  • the hook is correct. Did you exit after wp_redirect? Quoted from the codex "wp_redirect() does not exit automatically and should almost always be followed by exit. "
    – Mridul Aggarwal
    Oct 18, 2012 at 7:43
  • @MridulAggarwal - I sure did exit. Since the error originates in pluggable.php, it's clearly caused by the wp_redirect. It's a pity though that I don't get a stack trace.
    – Myself
    Oct 18, 2012 at 13:07
  • it sometimes happens that the filename that appears on the screen is not where the error originated. Check if you have some output before you call wp_redirect. Even a space before <?php is enough to create an error
    – Mridul Aggarwal
    Oct 18, 2012 at 13:37

1 Answer 1

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Wordpress provides a way to prevent the header HTML from being rendered, by appending &noheader=true to the url.

That will cause the header HTML to wait for you to call it manually, so that you can do a redirect before that.

To later render the header HTML from your page, you'll have to use this:

if ( isset($_GET['noheader']) ) {
    require_once(ABSPATH . 'wp-admin/admin-header.php');
}

For more information, read this article: WordPress and wp_redirect() function problem.

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