If you receive a PHP error similar to that above, you’ll need to enable output buffering if you can. This can be done fairly simply by adding the following line to your .htaccess file. (That lives in the root folder of your website: often named /htdocs, /httpdocs or /public_html.)

php_value output_buffering 4096

This instructs PHP not to send data to the browser until the full page has been read by the server. (Usually, PHP sends HTML to the browser, processes a PHP block when it comes to one, sends the next bit of HTML and so on.

If you try and instruct the browser to do something while it’s in this mode, unless the instruction is right at the beginning, it’ll be too late, because the browser’s already receiving data. Using the output_buffering setting means PHP waits for the entire HTML page to be parsed before sending the resulting HTML to the browser, with any instructions at the beginning of the page.)