gistfile1.php

<?php
function saveCacheFile($file, $contents)
{
    $len = strlen($contents);

    $cachefile_fp = @fopen($file, 'xb'); // x is the O_CREAT|O_EXCL mode
    if ($cachefile_fp !== false) { // create file
        if (fwrite($cachefile_fp, $contents, $len) < $len) {
            fclose($cachefile_fp);
            return PEAR::raiseError("Could not write $file.");
        }
    } else { // update file
        $cachefile_lstat = lstat($file);
        $cachefile_fp = @fopen($file, 'wb');
        if (!$cachefile_fp) {
            return PEAR::raiseError("Could not open $file for writing.");
        }

        $cachefile_fstat = fstat($cachefile_fp);
        if (
          $cachefile_lstat['mode'] == $cachefile_fstat['mode'] &&
          $cachefile_lstat['ino']  == $cachefile_fstat['ino'] &&
          $cachefile_lstat['dev']  == $cachefile_fstat['dev'] &&
          $cachefile_fstat['nlink'] === 1
        ) {
            if (fwrite($cachefile_fp, $contents, $len) < $len) {
                fclose($cachefile_fp);
                return PEAR::raiseError("Could not write $file.");
            }
        } else {
            fclose($cachefile_fp);
            $link = function_exists('readlink') ? readlink($file) : $file;
            return PEAR::raiseError('SECURITY ERROR: Will not write to ' . $file . ' as it is symlinked to ' . $link . ' - Possible symlink attack');
        }
    }

    fclose($cachefile_fp);
    return true;
}