Why ob_start() solves the header() error?

Question

I have a script that has a call to header(); and its working fine for a couple of days since I first started running the script.

then after a couple of days, it started to have an error saying it cannot modify header information.

Then I put ob_start(); and ob_end_flush(); upon googling the error and it works!

The error has gone but my question is that why it works without ob_start(); and ob_end_flush(); for a couple of days before?

I want to know the explanation behind this behavior.

btw, I call header() this way:

if(condition is true){
header('Location: anotherpage.php');
}

and I have a session_start(); at the beginning.

Did it start failing because you tried to output something before the `header();`? — SpacedMonkey, Dec 05 '12 at 12:45
read about `ob_` functions. but in few words: this functions stores ewrything what you output by your script, so when u set headers, you are sure that nothing was outputted before. — Stranger, Dec 05 '12 at 12:45
@TinkuRana I read it and its hard to comprehend why Im having that stranged behavior. — anagnam, Dec 05 '12 at 12:45
have u added session into action after couple of days because me too faced similar strange problem after adding session into action otherwise it was worked fine. — Dipesh Parmar, Dec 05 '12 at 12:45
php header function gives error if any output is there before header function. ob_start function stop any output from script — Tinku Rana, Dec 05 '12 at 12:47
@DipeshParmar yes I have a `session_start();` at the beginning. — anagnam, Dec 05 '12 at 12:52
here is ur ans http://stackoverflow.com/questions/1450990/why-ob-start-must-come-ahead-of-session-start-to-work-in-php — Dipesh Parmar, Dec 05 '12 at 12:54
ob_start() doesn't actually solve the problem, it works round it. To solve the problem, you need to identify what is being output before the headers and actually fix it — Mark Baker, Dec 05 '12 at 12:58

score 2 · Answer 1 · answered Dec 05 '12 at 12:45

2

Previously, you had no non-header output before the header line. Now, you have non-header output before the header line. That will only work if the output is buffered so the header can be actually output ahead of it.

answered Dec 05 '12 at 12:45

David Schwartz

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score 1 · Answer 2 · answered Dec 05 '12 at 12:45

1

You can not output any data before a header!

ob_start() is a output buffer and will buffer all echoed data and print it after all headers etc.

PHP flushes the data when the script is finished automatically so there's no need of the "ob_end_flush()"

answered Dec 05 '12 at 12:45

Niclas Larsson

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score 1 · Answer 3 · answered Dec 05 '12 at 12:46

You have some code outputting something before header() is called. It can be a print, echo or similiar statement, or even a white space before <?php.

The reason why ob_start causes the error to go away is because it causes any output to be buffered, therefore defered until the moment you call ob_flush.

This way it's guaranteed that the headers will come before content, even if you mess with the order of the commands in the code.

Why ob_start() solves the header() error?

3 Answers3