PHP:Decode-DP[Trj]

Using Firefox, Avast keeps giving me a warning about “PHP:Decode-DP[Trj]” whenver I visit a website that belongs to a friend.

I have searched the Internet (and Avast FAQ) without finding any information about this threat. The only specific reference I could find was on StackOverflow from June last year. They were dismissive about this.
http://meta.stackoverflow.com/questions/297315/stackoverflow-avast-detected-infection-phpdecode-de-trj

If this is a serious threat I want to warn my friend and get them to fix their site, if it’s not a serious threat, why is Avast flagging it up?

Any advice?

What is the link for that site ? (make it not clickable)

[b]If this is a serious threat[/b] I want to warn my friend and get them to fix their site, if it's not a serious threat, why is Avast flagging it up?
Impossible to answer unless you give the URL so we can check ....