You might receive an error message on your browser “Warning: Cannot modify header information – headers already sent by (output started at /path/wp_demo/wp-config.php:34)” right after upgrading your WordPress or a new WordPress installation.
Remove spaces, new lines, or other garbage in wp-config.php
Open wp-config.php from your root WordPress installation.
Now check and remove all spaces before <?php
and Scroll down to check and remove all spaces after ?>
*This error could happen on other files too. Read the error message carefully as it states the specific file name where the error occurred. For other file follow the same procedure.