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The location of the PHP error file is set in the php.ini file, for example:

error_log = /var/log/httpd/php_error.log

If you do not have access to the php.ini file in your hosting setup, you may be able to change the location of the PHP error log so that you can see the error messages. There are a couple of ways to to do that, for details see http://php.net/configuration.changes

If you are allowed to use .htaccess files in the document root directory, you can use the directive php_flag to set a local value to the error_log directive.

Otherwise, if you are handy with PHP, you may be able to set up a local value for the session using the ini_set function.

It is important to find out if a PHP error has happened and where it occurred before the problem can be addressed.