I made some changes to the phpsysinfo network module in the past, but I've taken that machine down, and I'm not using that funtionality of on my recent server.
I made a script, which were put in /etc/cron.monthly, which copied some network statistics data into a "safe" file, which were used to calculate the amount of data transferred each month.
The method I used could easily be put in /etc/cron.daily instead, and you would then be able to se the daily traffic.
Best regards
Per Smith