Wordpress Feeds: PHP Warning Cannot modify header information - headers already sent

Question

Every time a RSS feed (ie: (www.example.com/feed/) on my wordpress site is called I get a PHP E_Warning:

Cannot modify header information - headers already sent by (output started at /var/www/vhosts/(...)/httpdocs/wp-includes/functions.php:3464)

I did my research and most advice points towards blank spaces at the beginning and end of PHP files. But this is a wordpress core file.

ob_end_flush happens on line 3464:

function wp_ob_end_flush_all() {
    $levels = ob_get_level();
    for ($i=0; $i<$levels; $i++)
        ob_end_flush();
}

I did noticed that this functions.php did not have a closing ?> but given that it's a core wordpress file I would not question this too far :)

These errors usually don't have anything with the core files. I'd look in the theme functions.phpfile, to see if there is something there, related to the feed (add_feed(), add_action( 'rss2_item',...); or similar action, since you've said it happens only on your feed). — dingo_d

Mukul Medatwal Mukul Medatwal · Accepted Answer · 2016-02-26T13:14:18

This error is comes because when you are using header function and php get output before that function then php show that warning message.

so check that you are not using "echo" or not call any function before the header function that gives output. if so please remove that.

There is a tip for remove that kind of error.

please remove all the blank lines and spaces before the file. and put

ob_start(); at the beginning of the file.

i hope will work for you.

Thanks!!