Multiple file Upload without third party codes

If I remember correctly you can easily use CI's file uploader to work with multiple files, see below:

PHP Code:

$this->load->library('upload', $config);

// Cache the real $_FILES array, because the original
// will be overwritten soon :)
$files        = $_FILES;
$file_count    = count($_FILES['upload_field_name']['name']);

// Iterate over the $files array
for($i = 0; $i < $file_count; $i++)
{
    // Overwrite the default $_FILES array with a single file's data
    // to make the $_FILES array consumable by the upload library
    $_FILES['upload_field_name']['name']        = $files['upload_field_name']['name'][$i];
    $_FILES['upload_field_name']['type']        = $files['upload_field_name']['type'][$i];
    $_FILES['upload_field_name']['tmp_name']    = $files['upload_field_name']['tmp_name'][$i];
    $_FILES['upload_field_name']['error']        = $files['upload_field_name']['error'][$i];
    $_FILES['upload_field_name']['size']        = $files['upload_field_name']['size'][$i];

    if( ! $this->upload->do_upload('upload_field_name'))
    {
        // Handle upload errors
        
        // If an error occurs jump to the next file
        break;
    }
    else
    {
        // Successfull file upload
    }
} 


And on the upload field you should add an extra attribute multiple="multiple".

UPDATE: If you want to enable the upload of a single file you should check $_FILES['upload_field_name']['name'] is_array, if not then you should go the usual way.