Code:
<?php
$auth = 0;
if(isset($_SERVER['PHP_AUTH_USER']))
{

  $uname = $_SERVER['PHP_AUTH_USER'];
  $pwd = $_SERVER['PHP_AUTH_PW'];

   $fp = fopen("user.txt","r");
   while($line = fgets($fp))
   {
     $arr = explode(":",$line);
     $un =trim(arr[0]);
     $pw =trim(arr[1]);

     if($un == $uname and $pw = $pwd)
     {
...