If you receive such error in the error logs OR in the browser make sure output_buffering is set to On in the php.ini file on the server.
If you are on a su-exec/su-php enabled server, copy the original php.ini from the server under the account and make the changes.
What is output_buffering ?
Output buffering allows you to send header lines (including cookies) even
after you send body content, at the price of slowing PHP’s output layer a
bit. You can enable output buffering during runtime by calling the output
buffering functions. You can also enable output buffering for all files by
setting this directive to On. If you wish to limit the size of the buffer
to a certain size – you can use a maximum number of bytes instead of ‘On’, as
a value for this directive (e.g., output_buffering=4096).
