Random Image Script

                Description
This is your standard random image script that takes a slightly
different approach than most, namely using standard HTTP headers
instead of reading the file through the script.
Installation/Usage
For the simplest possible installation, just drop this code in a
directory with the images you want to rotate, and call it like you
would a normal image, for example:
Code
A Rotator Apart
  http://photomatt.net
    Inspired by Dan Benjamin  >  http://hiveware.com/imagerotator.php
    Latest version always at:
    http://photomatt.net/scripts/randomimage
*/
// Make this the relative path to the images, like "../img" or "random/images/".
// If the images are in the same directory, leave it blank.
$folder = '';
// Space seperated list of extensions, you probably won't have to change this.
$exts = 'jpg jpeg png gif';
$files = array(); $i = -1; // Initialize some variables
if ('' == $folder) $folder = './';
$handle = opendir($folder);
$exts = explode(' ', $exts);
while (false !== ($file = readdir($handle))) {
    foreach($exts as $ext) { // for each extension check the extension
        if (preg_match('/\.'.$ext.'$/i', $file, $test)) { // faster than ereg, case insensitive
            $files[] = $file; // it's good
            ++$i;
            }
        }
    }
closedir($handle); // We're not using it anymore
mt_srand((double)microtime()*1000000); // seed for PHP
header('Location: '.$folder.$files[$rand]); // Voila!
?>
Notes
For additional information about the thinking behind this script, see
A Better Image Rotator
http://photomatt.net/scripts/randomimage
$exts = 'jpg jpeg png gif';
$exts = explode(' ', $exts);
只是查看有没有以jpg jpeg png gif后缀的文件，比较笨，没看具体文件格式