function RandomFile($folder='', $extensions='.*'){
  // fix path:
  $folder = trim($folder);
  $folder = ($folder == '') ? './' : $folder;
  // check folder:
  if (!is_dir($folder)){ die('invalid folder given!'); }
  // create files array
  $files = array();
  // open directory
  if ($dir = @opendir($folder)){
    // go trough all files:
    while($file = readdir($dir)){
      if (!preg_match('/^\.+$/', $file) and 
        preg_match('/\.('.$extensions.')$/', $file)){
        // feed the array:
        $files[] = $file;        
      }      
    }    
    // close directory
    closedir($dir);  
  }
  else {
    die('Could not open the folder "'.$folder.'"');
  }
  if (count($files) == 0){
    die('No files where found :-(');
  }
  // seed random function:
  mt_srand((double)microtime()*1000000);
  // get an random index:
  $rand = mt_rand(0, count($files)-1);
  // check again:
  if (!isset($files[$rand])){
    die('Array index was not found! very strange!');
  }
  // return the random file:
  return $folder . $files[$rand];
}

//用法演示：
// "jpg|png|gif" matches all files with these extensions
print RandomFile('test_images/','jpg|png|gif');
// returns test_07.gif
// ".*" matches all extensions (all files)
print RandomFile('test_files/','.*');
// returns foobar_1.zip
// "[0-9]+" matches all extensions that just 
// contain numbers (like backup.1, backup.2)
print RandomFile('test_files/','[0-9]+');
// returns backup.7