Why i failed to called my config file

PHP
Why i failed to called my config file
below is my code

config.php
######LOG#########
$fileHandle = fopen("c:\ERI.log", "a");

Logic.php
#######LOG FILE#############
function LogFile($string)
{
require_once("config.php");
  $str = $string;
  if (fwrite($fileHandle, $str))
    echo "<br>Wrote to file \n </br>";
  else
    echo "write failed. \n";
fclose($fileHandle);
}
#######log testing to file#########
function Testing2()
{
$timestamp = time();
$string = date("l[H:i:s]", $timestamp)."stop process \r\n ";
return $string;
 }

$newERI->LogFile($newERI->Testing1());

if i write $fileHandle = fopen("c:\ERI.log", "a"); i received this error
Warning: fwrite(): supplied argument is not a valid stream resource in C:\wamp\www\testing.php on line 385
write failed.
Warning: fclose(): supplied argument is not a valid stream resource in C:\wamp\www\testing.php on line 389