Thanks - here is the result:
Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/website.ch/httpdocs/portfolio/log-in.php:6) in /home/httpd/vhosts/website.ch/httpdocs/webassist/mysqli/authentication.php on line 138
("website.ch" has been substituted for the actual server address)