PHP: how to know if output already started?

Question

I use the same php script for including and for ajax loading. It is some random catalog items which are loaded with the page and can be loaded with ajax. I need to use header() function there only if it is loaded via ajax.

When I use header function and the output already started I get php warning about it. How does php know that output already started? What is the best way to determine that, and not to call header function?

Thanks.

Answer 1

http://php.net/manual/en/function.headers-sent.php

// If no headers are sent, send one
if (!headers_sent()) {
    header('...');
    exit;
}

Answer 2

There's an easy built-in for that:

if (!headers_sent()) {
    header('Your header here');
}

Not much more to add :)