No previous output,but header redirect doesn't work in PHP

header('Location: ' . $url);

I've checked that there is no previous output:

ob_start();

...
var_dump(ob_get_contents());
ob_flush();
header('Location: ' . $url);

outputs:

string '' (length=0)

Then why does the redirect fail?

Though I see lots o开发者_如何学Cf warnings and notices in the error_log,but that doesn't affect header() as long as it doesn't output anything to the browser,right?

UPDATE

Some logs(should not be related though):

[Wed May 19 00:26:10 2010] [error] [client 127.0.0.1] PHP Deprecated:  Function eregi() is deprecated in D:\\Works\\general
[Wed May 19 00:26:10 2010] [error] [client 127.0.0.1] PHP Stack trace:, referer: http://localhost/cookie_usage.php
[Wed May 19 00:26:10 2010] [error] [client 127.0.0.1] PHP   1. {main}() D:\\Works\\login.php:0, referer: http://localhost/cookie_usage.php
[Wed May 19 00:26:10 2010] [error] [client 127.0.0.1] PHP   2. tep_redirect() D:\\Works\\login.php:33, referer: http://localhost/cookie_usage.php

ob_flush() sends to the output buffer. (prints what is in the OB) That is considered output. you may want to use ob_clean()

it is also good practice to use exit() or die() after a header('location ...') call.

Also note that whitespace at the end of the file will do this.

For the most part I never use ?> at the end of a PHP file anymore, just leave it open

?> is optional anyway.

I was facing similar issue before few days. There was extra white space in the end of php included file which I was calling before header function. I removed that white space and it starter working for me.

that doesn't affect header() as long as it doesn't output anything to the browser,right?

wrong.