You have been attacked through a Zeus backdoor, attack code is being changed all the time all of the time from bulletproof servers.
Avast is not your first line of defense here, your website software should be updated/upgraded, have secure plug-ins.
Server software should not give away too noisy details to attackers.
You should not be vulnerable to PHP attacks and you should have your website software fully patched and upgraded.
Attackers always go for the low hanging fruit, and when you have ripe apples in the shop window for them…

polonus