To enable PHP error logging, simply add the following lines to your website's main .htaccess file:
php_flag log_errors on
php_value error_log /var/www/vhtdocs/userwebXXXXX/php_error_log
Replace userwebXXXXX with your FTP user ID.
You can also use a different name to error_log for the filename. For example, if you have more than one website, you could use:
php_value error_log /var/www/vhtdocs/userwebXXXXX/websitename_php_error_log