8.19.1 Problem
You are trying to
send a HTTP header or cookie using header( ) or setcookie( ), but PHP reports a "headers already sent" error message.
8.19.2 Solution
This error happens when you send nonheader output before
calling header( ) or setcookie( ).
Rewrite your code so any output happens after sending
headers:
// good
setcookie("name", $name);
print "Hello $name!";
// bad
print "Hello $name!";
setcookie("name", $name);
// good
<?php setcookie("name",$name); ?>
<html><title>Hello</title>
8.19.3 Discussion
An HTTP message has a header and a body, which are sent to the
client in that order. Once you begin sending the body, you can't send any more
headers. So, if you call setcookie( ) after printing some HTML, PHP
can't send the appropriate Cookie header.
Also, remove trailing whitespace in any include files. When you include a
file with blank lines outside <?php ?> tags, the blank lines are
sent to the browser. Use trim( ) to remove
leading and trailing blank lines from files:
$file = '/path/to/file.php';
// backup
copy($file, "$file.bak") or die("Can't copy $file: $php_errormsg);
// read and trim
$contents = trim(join('',file($file)));
// write
$fh = fopen($file, 'w') or die("Can't open $file for writing: $php_errormsg);
if (-1 == fwrite($fh, $contents)) { die("Can't write to $file: $php_errormsg); }
fclose($fh) or die("Can't close $file: $php_errormsg);
Instead of processing files on a one-by-one basis, it may be
more convenient to do so on a directory-by-directory basis. Section 19.8 describes how to process all the files in a directory.
If you don't want to worry about blank lines disrupting the
sending of headers, turn on output buffering. Output buffering prevents PHP from
immediately sending all output to the client. If you buffer your output, you can
intermix headers and body text with abandon. However, it may seem to users that
your server takes longer to fulfill their requests since they have to wait
slightly longer before the browser displays any output.
