Can't change headers if there is an output?

Question

From what I have studied, headers cannot be changed after there is an output from a script. Why is it then that in this code sample that I wrote, the script has several outputs before it creates a new header, and yet everything works fine?

<?php
$name = "tommy" ;
?>

<?php 
headers_sent($filename, $linenum) ;
header("name: $name") ;
echo "tommy <br>" ;
echo "Headers  sent in $filename on line $linenum" ;
?>

In addition the output says that the headers were sent in line 0... how is that possible if I added a header afterwards?

Answer 1

Check if you have Output Buffering set to On in your php.ini. It might also be On by default on some PHP versions, according to this comment on php.net: http://www.php.net/manual/en/ref.outcontrol.php#69059

If that's the case, that's probably why your script executes fine with no errors, even though there is "output" before headers are sent.

EDIT: If you don't have access to the php.ini file itself, you can probably check the value of output buffering from your PHP script by calling ini_get('output_buffering');.

Answer 2

Output means, in this case, sending information to the client. Could be an echo, print_r or another form of outputting info in the screen.

There are more ways of outputting data, for example: setting a cookie will also send output. You can't set a cookie and proceed with header changes.

The error in your code: On line 3 you close php ?> and open it on line 5 <?php. Line 4 is output, a \n (newline-character). This will throw an error.
If you do not see that error, your error levels are probaly wrong. You can do an echo above your header, if you still get no errors, it's error_reporting.