You might received 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 same procedure.