如何在codeigniter中上传多个文件并重命名每个文件？

hey I want to upload multiple file using codeigniter.. i get successed about that but now i want to rename each file at the time of uploading.. I cant do this ..please help

here is my code..

if(isset($_FILES['userfile'])){
$config['upload_path'] = './resources/uploads/';
$config['allowed_types'] = 'gif|jpg|png';
$config['max_size'] = '1024';
$this->load->library('upload', $config);                                    
$files = $_FILES;
$cpt = sizeof($_FILES['userfile']['name']);

for($i=0; $i < $cpt; $i++)
{
$file_name=time().$i."_".($i+1);
$_FILES['userfile']['name']= $files['userfile']['name'][$i];
$_FILES['userfile']['type']= $files['userfile']['type'][$i];
$_FILES['userfile']['tmp_name']= $files['userfile']['tmp_name'][$i];
$_FILES['userfile']['error']= $files['userfile']['error'][$i];
$_FILES['userfile']['size']= $files['userfile']['size'][$i];  
$this->upload->do_upload(); 
}

I want this $file_name=time().$i."_".($i+1); for rename each file..

Than simply assign $file_name to file name as you required,

$file_name = time().$i."_".($i+1);
$_FILES['userfile']['name'] = $file_name;

To add extension with unique name,

$ext = end((explode(".", $files['userfile']['name'][$i])));
$file_name = time().$i."_".($i+1).".".$ext;
$_FILES['userfile']['name'] = $file_name;