function generateRandomString($length = 10) {   $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';   $randomString = '';   for ($i = 0; $i < $length; $i++) {     $randomString .= $characters[rand(0, strlen($characters) - 1)];   }   return $randomString; }echo generateRandomString(20);/** * 隨機生成字符串 * @param int $length * @return null|string */private static function getRandChar($length = 8){  $str = null;  $strPol = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz";  $max = strlen($strPol)-1;   for($i=0;$i<$length;$i++){    $str.=$strPol[rand(0,$max)]; //rand($min,$max)生成介于min和max兩個數之間的一個隨機整數  }   return $str;}