Thank you for your replies.
I've ended up doing the following for my PHP and Drupal logs:
Uncommented error_log = /var/log/php/php_errors.log in /etc/php.ini # mkdir /var/log/php # chown -R apache.apache /var/log/php
Appended following line to /etc/syslog.conf local6.* /var/log/php/drupal.log
In Drupal modules screen enabled Syslog module and diabled Database Logging module
In Drupal settings screen changed Syslog to LOCAL6
(And later I'll add rotation scripts under /etc/logrotate.d )
Regards Alex