The long-term answer is that all output from your PHP scripts should be buffered in variables. This includes headers and body output. Then at the end of your scripts do any output you need.
The very quick fix for your problem will be to add
ob_start();
as the very first thing in your script, if you only need it in this one script. If you need it in all your scripts add it as the very first thing in your header.php file.
This turns on PHP’s output buffering feature. In PHP when you output something (do an echo or print) it has to send the HTTP headers at that time. If you turn on output buffering you can output in the script but PHP doesn’t have to send the headers until the buffer is flushed. If you turn it on and don’t turn it off PHP will automatically flush everything in the buffer after the script finishes running. There really is no harm in just turning it on in almost all cases and could give you a small performance increase under some configurations.
If you have access to change your php.ini configuration file you can find and change or add the following
output_buffering = On
This will turn output buffering out without the need to call ob_start().
To find out more about output buffering check out http://php.net/manual/en/book.outcontrol.php