Saving a file locally, but comparing it to existing file first

[writer]

I'm using the following code to copy an XML feed locally, but I'd like it to first check the remote file to see if it's the same as the original file... if so, I don't need to copy it again.

 

<?php
$content = $_POST['content'];
$file = $_POST['fileName'];
$content = stripslashes($content);
$writeFile = fopen($file, "w");
if(fwrite($writeFile, $content))
{ echo $content; }
else { echo "Error writing to file"; }
fclose($writeFile);
?>

[writer]

At the heart of the matter: Can I use fopen() to compare two documents?