Cannot send headers; headers already sent

Question 1

Let's try to debug together. In Mage.php file line 766

$localConfigFile = $etcDir . DS . 'local.xml';

    if (is_readable($localConfigFile)) {
        $localConfig = simplexml_load_file($localConfigFile);
        date_default_timezone_set('UTC');
        if (($date = $localConfig->global->install->date) && strtotime($date)) {
            self::$_isInstalled = true;
        }
    }

So what Magento tries to do here is to load app/etc/local.xml

My guess would be that when you change your DB name/username/password you saved the file incorrectly (some editors add BOM signature). The local.xml file should be saved UTF8 without BOM option.

Question 2

many time this problem occuer from file permission

so please set proper file permission for more info

http://www.magentocommerce.com/wiki/groups/227/cache_is_not_writable

Question 3

enable error reporting and try to create log file

Mage::log('Cannot send headers; headers already sent in ' . $file . ', line ' . $line);

Question 4

Also one more reason of header already sent

In index.php added any echo then show error and in var/reports showing header already sent error

please check index.php if added any echo then remove it

Question 5

Yes, the problem for that error is only "]" for current issue. Try to check your new host detail, user name and password. Also, correct way to add.

Question 6

There has been an error processing your request

Cannot send headers; headers already sent in /var/www/leaveasy.com/Leaveasy/app/Mage.php, line 767:

Trace:

#0 /var/www/leaveasy.com/Leaveasy/lib/Zend/Controller/Response/Abstract.php(115): Zend_Controller_Response_Abstract->canSendHeaders(true)
#1 /var/www/leaveasy.com/Leaveasy/app/code/core/Mage/Core/Model/App.php(1246): Zend_Controller_Response_Abstract->setHeader('Content-Type', 'text/html; char...')
#2 /var/www/leaveasy.com/Leaveasy/app/code/core/Mage/Core/Controller/Varien/Front.php(80): Mage_Core_Model_App->getResponse()
#3 /var/www/leaveasy.com/Leaveasy/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(202): Mage_Core_Controller_Varien_Front->getResponse()
#4 /var/www/leaveasy.com/Leaveasy/app/code/core/Mage/Core/Controller/Varien/Front.php(172): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#5 /var/www/leaveasy.com/Leaveasy/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#6 /var/www/leaveasy.com/Leaveasy/app/Mage.php(684): Mage_Core_Model_App->run(Array)
#7 /var/www/leaveasy.com/Leaveasy/index.php(89): Mage::run('', 'store')
#8 {main}

Error log record number: 724004798

Question 7

I recently encountered this problem, and I was able to fix it myself. I reviewed my local.xml, and found out an extra character å character, because I accidentally entered and saved it when I pressed OPTION + A in my keyboard, instead of CONTROL + A. :)

Question 8

this can happen for several reasons. To find out wich of those reason is the one affecting you, follow these instructions:

Instead of opening your page in the browser, run in the commandline:

curl http://yourpage.com/

you will get the error and/or data that is being written before the headers can be send.

Example, I got this:

<br />
<b>Warning</b>:  file_put_contents(/var/www/html/var/log/system.log): failed to open stream: Permission denied in <b>/var/www/html/app/Mage.php</b> on line <b>832</b><br />
<br />
<b>Warning</b>:  chmod(): Permission denied in <b>/var/www/html/app/Mage.php</b> on line <b>833</b><br />
<script type="text/javascript">window.location.href = 'http://bluepowershop.localhost/errors/report.php?id=629963827323&skin=default';</script>

that means I have to setup my permissions for var/log files to be writtable.

Cannot send headers; headers already sent

What I Tried