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<?php

$folder = '';

$exts = 'jpg jpeg png gif bmp';

$files = array(); $i = -1;
if ('' == $folder) $folder = './';
$handle = opendir($folder);
$exts = explode(' ', $exts);
while (false !== ($file = readdir($handle))) {
    foreach($exts as $ext) {
        if (preg_match('/.'.$ext.'$/i', $file, $test)) {
            $files[] = $file;
            ++$i;
            }
        }
    }
closedir($handle);
mt_srand((double)microtime()*1000000);
$rand = mt_rand(0, $i);

header('Location: '.$folder.$files[$rand]);
?>