<?php
$filename = 'iknowit.txt';
$somecontent = $_GET['w']."\n";

// Let's make sure the file exists and is writable first.
if (is_writable($filename)) {

    if (!$handle = fopen($filename, 'a')) {
         echo "Cannot open file ($filename)";
         exit;
    }

    // Write $somecontent to our opened file.
    if (fwrite($handle, $somecontent) === FALSE) {
        echo "Cannot write to file ($filename)";
        exit;
    }

    fclose($handle);

} else {
    echo "The file $filename is not writable";
}
?>