What the white screen of death means
From Online Manual
What is the White Screen of Death
A white screen when opening a page in SMF (or in any other PHP-based application) means that an irrecoverable error occurred, which caused the execution of the script to be halted before any content is outputted to your browser.
There are mainly two reasons for this to happen. The first is that there is a PHP error in the code of your forum (which you might get if you are performing changes in the code or installing a MOD). The second is that PHP cannot use the necessary memory for the task being requested.
Why Do I See a White Page and Not the Error Itself?
Your PHP configuration may prevent this error from being displayed. On a live and fully functional web site your guests are not supposed to see errors (if they occur), because they might reveal file paths or other sensitive information. For this reason, some hosts specifically configure PHP to not show errors.
How to Get More Information
There are several ways to obtain more information. Some of these are listed below, but please be aware that they might not exist in your specific server configuration.
Contact your Host
Assuming you do not run your own server, contacting your host is the easiest way to obtain information. Your host should have access to all your logs, so they should be able to provide you with detailed information.
Check your Logs
When a PHP error occurs, it is logged in a file in your server's hard drive. Depending on your configuration, you should also have that file in your home directory. Use your FTP client and search for a file which is usually called "error_log", "error.log", or variations of this. If you do not find such file, that is because your server is not configured to create it. Contact your host and ask them to enable this option, as it is a very helpful debug tool.
Try to Output More Information
It is possible to tell PHP to output the error information directly to your browser. To do so, edit SMF's index.php (make sure that you create a backup first) with your favourite text editor and find:
// Get everything started up... define('SMF', 1);
error_reporting(E_ALL); ini_set('display_errors', 1); // Get everything started up... define('SMF', 1);
After you fix the underlying problem you should revert this change.
How to Solve the Problem
How to solve the problem depends specifically on what the problem is. The most common causes are:
PHP Code Errors
You should only see these errors when you are editing your forum's code or when installing/uninstalling MODs. Your log should display something like (these are just examples):
Parse error: syntax error, unexpected $end in /home/user/public_html/forum/index.php on line 175
Fatal error: Call to undefined function getInitialInformation() in /home/user/public_html/forum/index.php on line 175
(error, file name, file path and and line number might of course vary)
PHP Memory Errors
This error occurs when the code execution tries to allocate more memory than the system is configured to grant. Your log should tell you something like:
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 826064718 bytes) in /home/user/public_html/forum/index.php on line 175
(memory quantities, file name, file path and and line number might of course vary)