Usually, getting such errors on your WordPress site or in the error_log file means that you have extra spaces or lines before or after the opening and closing PHP tags, <?php and respectively ?> . This happens mostly to files from the current theme.
In the error_log file you will see entries like:
[30-Jan-2020 10:55:08 UTC] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/site1/public_html/wp-content/themes/BlogTheme/functions.php:1) in /home/site1/public_html/wp-content/themes/BlogTheme/functions.php on line 873
[30-Jan-2020 10:55:31 UTC] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/site1/public_html/wp-content/themes/BlogTheme/functions.php:1) in /home/site1/public_html/wp-content/themes/BlogTheme/functions.php on line 873So, to fix these PHP warnings, you must edit the file (e.g functions.php) and double-check for extra lines/spaces. In our case, the warnings were caused by a space before the opening PHP tag:
You can edit the file directly from your File Manager in cPanel/Plesk/DirectAdmin etc or you can download the file, edit it and upload it.
Open the WordPress site again and check for any new PHP messages.