cant access vars within function?

[Jmacp]

<?php

   $a= $HTTP_GET_VARS['a'];
    $b= $HTTP_GET_VARS['b'];
    $c= $HTTP_GET_VARS['c'];
    $d= $HTTP_GET_VARS['d'];
    $e= $HTTP_GET_VARS['e'];
    $f= $HTTP_GET_VARS['f'];
    $g= $HTTP_GET_VARS['g'];


 function writeout($msg){
   $filename = "files/" . $a . "tmp.html";

    $fh= fopen($filename, 'a') or die("can't open file");
            
    if (filesize($filename) ==0) {
    
    //add the top stylesheet
    $SS = "files/css.txt";
    $Style= fopen($SS, 'r') or die("can't open file");
    $read = fread($Style, filesize($SS)); 
    fwrite($fh, $read);
    fclose($Style);
    }

   $b = str_replace("&","&amp;",$b);
   $c = str_replace("&","&amp;",$c);
   $a = str_replace("&","&amp;",$a);
   $d = str_replace("&","&amp;",$d);
   $e = str_replace("&","&amp;",$e);
   $f = str_replace("&","&amp;",$f);
   $g = str_replace("&","&amp;",$g);

   $newline = "\r\n";
 
  fwrite($fh, $newline."<table border=2 width=" .chr(34)."5%".chr(34).">");
  fwrite($fh,"<tr class=".chr(34)."hdr".chr(34).">");
  fwrite($fh,"<td>".$a."</td>");
  fwrite($fh, $newline."<a href="."http://www.site.com/".$msg." "."target=" .chr(34). "_blank".chr(34).">"."username"."</a></td>");
  fwrite($fh,"<td></td></tr>");
 fclose($fh);
}


writeout($a);
writeout($b);
writeout($c);
writeout($d);
writeout($e);
writeout($f);
writeout($g);

?>

giving me undefined variable a,b,c,d,e,f,g etc.. as if i dont have access to the vars within the fucntion?