From the online documentation:

$fU - From URI username

$fU - reference to username in URI of 'From' header

It is R/W variable (you can assign values to it directly in configuration file)

But i am no able to assign values to $fU.  Has anyone experienced this?

Snippet of code in the configuration file of mine and its respective logs showing that $fU is not getting overwritten.  I have tried to hard code values to it assign variables to it, all to no avail.  

configuration file code snippet:
  
  xlog("L_INFO", "[$ci]   from header username pre mod: $fU");
  $fU = "5555555555";
  xlog("L_INFO", "[$ci]   from header username post mod: $fU");

Logfile snippet:

INFO: (s) [7EAB8FECD723E8444E36C659F331068B41EC6BB0]   from header username pre mod: 3224567880
INFO: (s) [7EAB8FECD723E8444E36C659F331068B41EC6BB0]   from header username post mod: 3224567880

Thanks,

Karthik