function generatePassword ($length = 8) {
  $password = "";
  $possible = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-$!?]["; 
 
  $i = 0; 
  // add random characters to $password until $length is reached
  while ($i < $length) { 
    // pick a random character from the possible ones
    $char = substr($possible, mt_rand(0, strlen($possible)-1), 1);
    // we don't want this character if it's already in the password
    if (!strstr($password, $char)) { 
      $password .= $char;
      $i++;
    }
  }
  return $password;
}