为什么收到警告：无法修改标头信息-标头已经发送过（输出从...开始）？

Question

I'm reusing a search PHP script, but I get this error:

Warning: Cannot modify header information - headers already sent by (output started at /Volumes/Work/MAMP/PC Flag/php_scripts/conection.php:2) in /Volumes/Work/MAMP/PC Flag/includes/search.php on line 63

line #2 conection.php:

$con = mysqli_connect("localhost", "root", "root", "cms");

line #63 search.php:

header('Location: search-page.php?keywords=' . urlencode($keywords));

What is the problem?

Also I need to mention that the script doesn't redirect me anymore to search-page.php as it should.

Answer 1

When you use header() it shouldn't be any output before, not even a space, you have two solution:

Dirty one

Use ob_clean() before the header to clean the output buffer

ob_clean();
header('Location: search-page.php?keywords=' . urlencode($keywords));

Correct one

Search in your script and your request route for any output, content before <?php , echo es, HTML code, print s or spaces after the ?> are usually the cause of this ;)

Debug

This snippet may help you to find out where is the output in your code:

var_dump(ob_get_contents());die()

Put this before line 63

Javascript workaround

In the case that everything fails, you have another option, use javascript to redirect, although I recommend you to keep trying without get to this point :)

Replace the header(...) with this:

echo '<script>window.location.href="search-page.php?keywords=' . urlencode($keywords)) . '";</script>';

Answer 2

You have send a header already as error states. Please investigate when and how headers are sent .

That usually happens when something is printed before that line to output buffer.

Make sure that you are using UTF-8 without BOM document encoding - BOM sign is invisible in most text editors yet can be interpreted as content which forces sending HTTP headers.

You can debug your output with Output Control function

Answer 3

It's happening because output has been sent already. You can't edit Header Information when you have echo'd anything!

Answer 4

It seems like the error you're getting is because an output have been occured.you can fix it with this code :

if (headers_sent()) {
    ob_clean();
}  

Just found this detailed answer .