unset()ting a variable passed by reference does not destroy this variable, but rather protects it's value against being further altered inside the function.

< ? php
  function subroutine(&$byname, $assign, $unset)
  {
     if($assign) $byname = 'set by subroutine';
     if($unset)  unset($byname);  
     $byname = 'finally set inside subroutine';  
  }

   $byname = 'Set Outside!';
   subroutine($byname, TRUE, FALSE);
   echo ('<p>   1st. call: ($assign=TRUE,  $unset=FALSE) returns $byname="'.$byname.'"</p>');

   $byname = 'Set Outside!';
   subroutine($byname, FALSE, FALSE);
   echo ('<p>   2nd. call: ($assign=FALSE, $unset=FALSE) returns $byname="'.$byname.'"</p>');

   $byname = 'Set Outside!';
   subroutine($byname, TRUE, TRUE);
   echo ('<p>   3rd. call: ($assign=TRUE,  $unset=TRUE)  returns $byname="<b>'.$byname.'</b>"</p>');

   $byname = 'Set Outside!';
   subroutine($byname, FALSE, TRUE);
   echo ('<p>   4th. call: ($assign=FALSE, $unset=TRUE)  returns $byname="<b>'.$byname.'"</p></b>');
? >

Outputs:

   1st. call: ($assign=TRUE,  $unset=FALSE) returns $byname="finally set inside subroutine"
   2nd. call: ($assign=FALSE, $unset=FALSE) returns $byname="finally set inside subroutine"
   3rd. call: ($assign=TRUE,  $unset=TRUE)  returns $byname="set by subroutine"
   4th. call: ($assign=FALSE, $unset=TRUE)  returns $byname="Set Outside!"

More information and a live Demo can be obtained from 
http://purodha.hostsharing.de/hostsharing/menu0/test_by_name.php