[Mornagest]

Well that doesn't work as the file is not created, even after restarting apache2.

I put this in /etc/php5/apache2/php.ini:

; Besides displaying errors, PHP can also log errors to locations such as a
; server-specific log, STDERR, or a location specified by the error_log
; directive found below. While errors should not be displayed on productions
; servers they should still be monitored and logging is a great way to do that.
; Default Value: Off
; Development Value: On
; Production Value: On
; http://php.net/log-errors
log_errors = On
error_log = /var/log/phptesterror.log

but the file /var/log/phptesterror.log doesn't exist, even after trying the diagnostic mail.