Error: HTTP headers have already been sent! Caused by output from file ~/forum/forum.php at line 120. The error was detected after initialisation.

Description: HTTP headers are used to give the web browser information on the content type or how to handle it. These headers are immediately sent to the browser as soon as text content is generated by the PHP script. At this place, the application expected the headers not to be sent yet and needs to add more headers but cannot do so because somewhere else output was generated. This likely happens when custom page elements have been inserted before this code or empty lines or UTF BOMs are present before the opening <?php tag. Please check the above code line and remove the output from there to resolve the error.