[PHP] fwrite()ing predefined variables

[Alberto Brea]

Hi,
Could somebody please tell me why I can't print pre-defined variables to a file with 
fwrite()?

The code I use is this:
 $filename = '../vlog.txt';
 $fd= fopen($filename, 'a');
  $a= date(YmdHi);
  $b= basename($PHP_SELF);
  $c= $HTTP_USER_AGENT;
  $d= $HTTP_HOST;
  $e= $_SERVER['HTTP_HOST'];
  $f= "xxx";
  $g= "yyy";
  fwrite($fd, "$a#$b#$c#$d#$e#$f#$g\n");
 fclose($fd);

And the output I get in the target file is this:
200212101552####localhost#xxx#yyy

Thanks a lot in advance

Alberto