add the contents of 2 txt files together - php

Question

I have this small piece of code on my website that i use to count downloads.
its pretty simple really,
counter.php sends the command and counter.txt is just a 1 line text file with a number that auto goes up every time the link is clicked.
My question is, is it possible to have 2 counter.txt files and add them to a third counter.txt file? so it would look something like:
counter.txt + counter2.txt = counter3.txt ?
$counter = 'counter.txt';
$download = 'downloadurlhere';
$number = file_get_contents($counter); // read count file
$number++; // increment count by 1
$fh = fopen($counter, 'w'); // open count file for writing
fwrite($fh, $number); // write new count to count file
fclose($fh); // close count file
header("Location: $download"); // get download
So essentially i want to offer 2 downloads
a light version and a full version
and then keep track of each download count separately. but then also have a count for the total of both downloads.
oh and to make sure i include enough detail, on the download.php page i echo the counter.txt file with
<?php echo file_get_contents('counter.txt');?>

Answer 1

Adding the counters of two files should be easy. You just have to read each file and then the variables are added and written to third counter file or whatever you desire. An example:
<?php
$first=file_get_contents("counter1.txt");
$second=file_get_contents("counter2.txt");
$sum=$first+$second;
file_put_contents("counter3.txt",$sum);
?>

Answer 2

So here was the end result, I had to swap around the txt files for counter.txt and urls but this worked perfectly...
$count = 'counterfull.txt';
$total = 'total.txt';
$download = 'URL';
$number1 = file_get_contents($count);
$number1++;
$fh = fopen($count, w);
fwrite($fh, $number1);
fclose($fh);
$number2 = file_get_contents($total);
$number2++;
$fh = fopen($total, w);
fwrite($fh, $number2);
fclose($fh);
header("Location: $download");