How to handle php error : Undefined offset 1 - php

Question

I've spent quite a lot of time looking on how to handle "Undefined offset 1" error, but couldn't resolve my problem.
I'm trying to create a one time downloadable URL to be used on my site and recently I've found how to do it using php. The code seems to work, but I'm getting an error like:Notice: Undefined offset: 1. As I'm really new to php, I can't figure out how to solve this problem.
Could you please help me!
<?php
/* Retrive the given token: */
$token = $_GET['q'];
if( strlen($token)<32 )
{
die("Invalid token!");
}
/* Define the file for download: */
$secretfile = "file.txt";
/* This variable is used to determine if the token is valid or not: */
$valid = 0;
/* Define what file holds the ids. */
$file = "urls.txt";
/* Read the whole token-file into the variable $lines: */
$lines = file($file);
/* Truncate the token-file, and open it for writing: */
if( !($fd = fopen("urls.txt","w")) )
die("Could not open $file for writing!");
/* Aquire exclusive lock on $file. */
if( !(flock($fd,LOCK_EX)) )
die("Could not equire exclusive lock on $file!");
/* Loop through all tokens in the token-file: */
for( $i = 0; $lines[$i]; $i++ )
{
/* Is the current token the same as the one defined in $token? */
if( $token == rtrim($lines[$i]) )
{
$valid = 1;
}
/* The code below will only get executed if $token does NOT match the
current token in the token file. The result of this will be that
a valid token will not be written to the token file, and will
therefore only be valid once. */
else
{
fwrite($fd,$lines[$i]);
}
}
/* We're done writing to $file, so it's safe release the lock. */
if( !(flock($fd,LOCK_UN)) )
die("Could not release lock on $file!");
/* Save and close the token file: */
if( !(fclose($fd)) )
die("Could not close file pointer for $file!");
/* If there was a valid token in $token, output the secret file: */
if( $valid )
{
header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename="'.basename($secretfile).'"');
header('Expires: 0');
header('Cache-Control: must-revalidate');
header('Pragma: public');
header('Content-Length: ' . filesize($file));
readfile($secretfile);
/*readfile($secretfile);*/
}
else
{
print "Invalid URL!";
}
?>
The error is on this line of code:
/* Loop through all tokens in the token-file: */
for( $i = 0; $lines[$i]; $i++ )
Thanks in advance for your help!