well, you'll basically have to iron out the paths of the directories.
you could try things like
../logs/mod.log
but it is really hard to say.
for opening a file and displaying its contents, a simple method would be:
<?php
$fp = fopen($filename, "r");
OR die("Could not open '" . $filename . "' for reading!");
$contents = fread($fp, filesize($filename));
fclose($fp);
echo '<textarea name="file_contents" rows="6" cols="60">'
echo htmlspecialchars($contents);
echo '</textarea>';
?>