[Users] question about avp

[unplug]

Say I have a record in the table usr_preferences with username,
attribute=function, value=voicemail.  I use
avp_db_load("$from/username", "s:function") to get the value.  I can
print out the value in xlog function by using $avp(s:function).  How
can I put that value in the header by using append_hf?  As I found
that it doens't work if I use
append_hf("P-hint: $avp(s:function)\r\n").  In other words, how can I
get back the value of the variable stored in avp for other functions
processing?