/** 
 * 生成32位以上唯一字符串ID 
 * @param int $length 不含前缀的长度，最小14，建议16+ 
 * @param string $prefix 前缀 
 * @return string 
 */
function unique_id($length = 32, $prefix = ''){    
    $only = $prefix;   
    $addLength = $length - 13;    
    $only .= uniqid();    
    if (function_exists('random_bytes')) {        
        $only.= substr(bin2hex(random_bytes(ceil(($addLength) / 2))),0,$addLength);   
    } elseif (function_exists('openssl_random_pseudo_bytes')) {       
        $only.= substr(bin2hex(openssl_random_pseudo_bytes(ceil($addLength / 2))),0,$addLength);   
    } else {        
        $only.= mt_rand(1*pow(10,($addLength)),9*pow(10,($addLength)));    
    }   
    return $only;
}