Magento white page at front and backend - php

Question

After customizing some files last week, both the back end and front end of the site I’m developing for went blank. I have already searched the internet and tried out some possible solutions, but none of them works.
Somehow I can’t manage to show any error message. I already uncommented ini_set(’display_errors’, 1);, changed error_reporting(E_ALL | E_STRICT); into error_reporting(E_ALL); and I tried even more possible solutions.
I also edited the memory to 128M, changed some settings in php.ini, changed local.xml.sample to local.xml. Nothing seems to work.
I seriously have no idea how I can solve this, so that’s why I’m here. I hope someone can help me. If you need more information, please ask, I don’t really know what’s necessary to answer my question.
After adding the following lines to index.php I managed to get error messages:
error_reporting(E_ALL);
ini_set(’display_errors’, 1);
ini_set(‘memory_limit’, ’256M’);
The error I get is:
Cannot send headers; headers already sent in /home/cheapas/public_html/index.php, line 39
Trace:
#0 /home/cheapas/public_html/lib/Zend/Controller/Response/Abstract.php(115): Zend_Controller_Response_Abstract->canSendHeaders(true)
#1 /home/cheapas/public_html/app/code/core/Mage/Core/Model/App.php(1246): Zend_Controller_Response_Abstract->setHeader(’Content-Type’, ‘text/html; char...’)
#2 /home/cheapas/public_html/app/code/core/Mage/Core/Controller/Varien/Front.php(80): Mage_Core_Model_App->getResponse()
#3 /home/cheapas/public_html/app/code/core/Mage/Core/Model/Url/Rewrite.php(204): Mage_Core_Controller_Varien_Front->getResponse()
#4 /home/cheapas/public_html/app/code/core/Mage/Core/Controller/Varien/Front.php(166): Mage_Core_Model_Url_Rewrite->rewrite()
#5 /home/cheapas/public_html/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#6 /home/cheapas/public_html/app/Mage.php(683): Mage_Core_Model_App->run(Array)
#7 /home/cheapas/public_html/index.php(91): Mage::run(’’, ‘store’)
#8 {main}
Guess what, line 39 is: ini_set(‘memory_limit’, ’256M’);
When I delete this line I get a blank page again, so that doesn’t solve anything. Does anyone have an idea on solving this? Thanks!
As you may have understood, I actually can get error messages to show when I force an error by adding an unknown or uncomplete line to my index.php. Somehow there is a problem which doesn’t want to show up.