I go through a file using
WHILE(! feof()), during which
I write to a file ($wfile).

Once done, I need to make a change
in the last line of the output file.

How do I do this?

The simplest way to do this is using the file(http://uk2.php.net/manual/en/function.file.php) function:

PHP Code:

// taken from: http://stackoverflow.com/questions/1062716/php-returning-the-last-line-in-a-file
$file = "/path/to/file";
$data = file($file); // this will return an array of the data
$line = $data[count($data)-1]; // this will return the last child in the array