Header Error

Header Error

From time to time I have been running into a problem where I get a message:

"Warning: Cannot modify header information - headers already sent by..."

Some people told me it has to do with white spaces before or after the php tags, but I have double checked that and it is not the case. In one case I simpl deleted an entire routine that logged some text to a file and that fixed it. However, I would better like to understand why it was causing an error. It also was not causing an error when I first ran it, but later caused an error. I am wondering if my host changed settings that caused the problem?

That error message is given if you attempt to change response headers in any way (for example with a header redirect) after some kind of data has already been sent as body (the data which appears as a web site's source in a browser). This could include PHP warnings if your host turned up the default strictness of the reporting, which could explain why it seemed to occur suddenly by itself.

Warning: Cannot add header information

Warning: Cannot add header information - headers already sent by (output started at file.php:23)

Have you ever gotten that warning message in PHP? Wonder what it's all about? Well, we can cover this very easily.

You are trying to send header information AFTER you have sent output to the browser.

Okay, lets give you an example of what will give that error.

php:
<?php
echo "hello";
header("Location: http://www.phpcomplete.com");
?>


This will raise an error, because you outputted hello to the browser via echo BEFORE you used the header() function. The header() function sends header information to the browser, so it makes sense the header inforation would come BEFORE the body, or the actual document. As soon as you send ANY content or data to the browser that isn't header information, you can't go back and send header information.

This would also give you an warning:

php:


<?php
header("Location: http://www.phpcomplete.com");
?>


While it may seem like the above isn't sending any data other than header information, the problem is in the spaces BEFORE the opening PHP tag. That is a problem, because that is actual data.

People seem to get confused most often with header() because they think they can do something like this:

formTest.php
php:
<?php
// Form has been submitted...
if ( isset($_POST?'submit']) ) {
if ( empty($_POST?'emailAddress'])) {
echo "You didn't fill in your email address.";
header("Location: formTest.php");
} else {
echo "Email was filled in.";
header("Location: emailOkay.php");
}
}
?>
<form method="post" action="formTest.php">
Enter an Email: <input type="text" name="emailAddress" />
<input type="submit" name="submit" value="Submit Email Address" />
</form>



emailOkay.php
php:
Your email was okay.


This seems to be a common problem amongst newbies. For some reason, they think that they can output data to the browser, and then send header information. Unfortunately, they are confusing the HTML meta-refresh tag with the PHP header() function.

The above can be rewritten as such:

formTest.php
php:
<?php
// Form has been submitted...
if ( isset($_POST?'submit']) ) {
if ( empty($_POST?'emailAddress']) {
header("Location: emailOkay.php");
} else {
echo 'Email was not entered.';
}
}
?>
<form method="post" action="formTest.php">
Enter an Email: <input type="text" name="emailAddress" />
<input type="submit" name="submit" value="Submit Email Address" />
</form>



emailOkay.php
emailOkay.php
php:
Your email was okay.


The above example displays how easy it is to redirect using the header() function along with forms.

Another problem newbies face with the header() function is the infamous error that occurs in weird places that aren't echo'ed out, for example:

examplePage.php
php:
<?php
include \"databaseInit.php\";
// some normal non-display code here
header(\"Location: nextStep.php\");
?>


databaseInit.php
php:

<?php
mysql_connect('localhost', 'username', 'password');
mysql_select_db('database');
?>



Here, the problem is in databaseInit.php, and it has to do with the blank lines. The blank lines before the code starts, and after the code end - both will cause problems. Remember, NO output whatsover, blank lines included!

Now, you may be feeling like that is a lot of work to use header(), but their is an easy solution to all of this: output buffering.

Output buffering is not only easy to use, but it works miracles. All the above examples could use output buffering, and would NOT cause errors, and would basically work. So you probably want to know how to use output buffering, so I will tell you.

In the above example with the database connections, we could do something like this:

examplePage.php
php:
<?php
include \"databaseInit.php\";
// some normal non-display code here
echo \"Stuff happens here\";
header(\"Location: nextStep.php\");
// other code for other things...
include 'cleanUpModule.php';
?>


databaseInit.php
php:
<?php
ob_start() // turn output buffering on
mysql_connect('localhost', 'username', 'password');
mysql_select_db('database');
?>



cleanUpModule.php
php:
<?php
ob_end_flush();
?>


Note: I never tried it, but I am told ob_end_flush() is called implicitly at the end of the PHP script, so you wouldn't even need to call ob_end_flush() at the end here if that is true.

Anyways, basically, output buffering doesn't send anything out to the browser until you tell it to, so you can have ouput and headers in whatever order you want. This is useful for many reasons, but even more useful in large projects when you don't know if someone is output something. The above would work, reordering the echo to happen after the header() function (of course, echo would never run, because header() would change the page location).

Well, that's it, I hope this clears up some problems. On a short note, everything here also applies to sessions and working with cookies. Once you send output, you can set a session or change cookies values. The methods here apply in the same manner though.

Enjoy!

Yeah it seems to come and go. What constitutes body data? Does output to a file count as data? In the one case, I was actually only writing to the file. There was no echo or anything like that, except to a text file for logging. Is that enough to get the error?

So does it make sense to use includes to avoid this problem? Is that all it takes?

It will be more helping if you post your code here for checking as i also can try it on my machine for further errors !