PHP Error Handling
Eliminating “headers already sent” Errors
Problem
You are trying to send an HTTP header or cookie using header() or setcookie(), but PHP reports a “headers already sent” error message.
Solution
This error happens when you send nonheader output before calling header() or set cookie().
Rewrite your code so any output happens after sending headers:
<?php
// good
setcookie("name", $name);
print "Hello $name!";
// bad
print "Hello $name!";
setcookie("name", $name);
// good
setcookie("name",$name); ?>
<html><title>Hello</title>
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. Describes how to process all the files in a directory.
Another perfectly legitimate approach to ensuring included files don’t have any trailing whitespace is to just leave off the closing ?> tag. If the included file is purely PHP, this method guarantees that you won’t have to go back to that file to clean up inadvertent whitespace.
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 because they have to wait slightly longer before the browser displays any output.
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