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.

score 25 · Accepted Answer · answered Jan 20 '11 at 21:19

25

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

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

answered Jan 20 '11 at 21:19

Jake

775
9
17

Thanks a lot! Just what I needed. And by the way why exit? – foreline Jan 20 '11 at 21:31
1

The use of exit is straight from the referenced PHP manual for a 'header: Location' call which requires it. In your case you should not add it, or you will break the output. – tomwalsham Jan 20 '11 at 21:33
Yeah sorry, I copied it straight from the manual and forgot to remove it :) – Jake Jan 20 '11 at 21:41

score 6 · Answer 2 · answered Jan 20 '11 at 21:20

6

There's an easy built-in for that:

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

Not much more to add :)

answered Jan 20 '11 at 21:20

tomwalsham

781
4
4

score 3 · Answer 3 · answered Jan 20 '11 at 21:20

3

One option is to call ob_start() at the beginning, which buffers all your output. That way you can send headers any time.

answered Jan 20 '11 at 21:20

Tesserex

17,166
5
66
106

score 2 · Answer 4 · answered Jan 20 '11 at 21:21

2

headers_sent() returns true if you cannot send additional headers.

answered Jan 20 '11 at 21:21

Floern

33,559
24
104
119

score 0 · Answer 5 · answered Jul 22 '14 at 04:06

0

Just remove the empty line before the code starts and add exit; after your output ends.

answered Jul 22 '14 at 04:06

Elangathir

41
2

There's no code in the question - how do you know what's in there and what isn't? Most of the current answers suggest using `headers_sent()` - is there a reason why your suggestion might be better? – andrewsi Jul 22 '14 at 04:28

score 0 · Answer 6 · answered Jan 20 '11 at 21:22

If you sent anything to client, the output started.

For example even a single html tag, it's already output.

You can either structure your application so that this doesn't happen, or you can turn on output buffering:

http://php.net/manual/en/function.ob-start.php

PHP: how to know if output already started?

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