If your error_log directive is unset errors will be written in your Apache log file for current VirtualHost. So double check your PHP configuration (php.ini) or write a simple page with phpinfo()
If this is true, you should look inside your code (may be even into index.php).
Pay attention to this: usually there are two separate php.ini files for Apache /etc/php5/apache2/php.ini
and CLI configuration /etc/php5/cli/php.ini
.
Please also consider that, if you want change your PHP configuration, you can use ini_set function.
ini_set('error_log', '/var/log/php/error_new.log');
Remember: the destination directory must exist and your web server (or php engine) must have all permission to write into.