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.

Jake · Accepted Answer · 2011-01-20 21:19:54Z

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, 2011 at 21:19

Jake

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3 Comments

foreline Over a year ago

Thanks a lot! Just what I needed. And by the way why exit?

tomwalsham Over a year ago

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.

Jake Over a year ago

Yeah sorry, I copied it straight from the manual and forgot to remove it :)

tomwalsham · Accepted Answer · 2011-01-20 21:20:43Z

6

There's an easy built-in for that:

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

Not much more to add :)

answered Jan 20, 2011 at 21:20

tomwalsham

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Comments

Tesserex · Accepted Answer · 2011-01-20 21:20:14Z

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, 2011 at 21:20

Tesserex

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Comments

Floern · Accepted Answer · 2011-01-20 21:21:04Z

2

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

answered Jan 20, 2011 at 21:21

Floern

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Comments

CodeVirtuoso · Accepted Answer · 2011-01-20 21:22:10Z

0

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

answered Jan 20, 2011 at 21:22

CodeVirtuoso

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Comments

Elangathir · Accepted Answer · 2014-07-22 04:06:29Z

0

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

answered Jul 22, 2014 at 4:06

Elangathir

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1 Comment

andrewsi Over a year ago

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?

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