No-CMS upload file technique to avoid duplicate file name

register.php

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');
// you can also extends CI_Controller for normal CodeIgniter code
class Register extends CMS_Controller{

        // this is the upload path
        $upload_path = BASEPATH.'../modules/ssc/assets/upload/';
	
        // this is how to use it
	public function upload(){
		$file = $_FILES['user_file'];
		$tmp_name = $file['tmp_name'];
		$original_name = $file['name'];
		$prefix = 'document';
		$new_name = $this->upload_file($tmp_name, $original_name, $prefix);
		// $new_name will be something like 'document_20120809201007_Ac8u_something.jpg
	}
	
	private function rand_string( $length ){
		$chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";	
	
		$size = strlen( $chars );
		$str = '';
		for( $i = 0; $i < $length; $i++ ){
			$str .= $chars[ rand( 0, $size - 1 ) ];
		}	
		return $str;
	}
	
	private function upload_file($tmp_file_name, $original_file_name, $prefix){
		// check upload path
		$upload_path = $this->upload_path;
		if(!file_exists($upload_path)){
			mkdir($team_directory);
		}
		if(!is_writable($upload_path)){
			chmod($upload_path, 0777);
		}
		
		$time_stamp = date('YmdHis');
		$new_file_name = $prefix.'_'.$time_stamp.'_'.$this->rand_string(4).
			'_'.$original_file_name;
		
		move_uploaded_file($tmp_file_name, $upload_path.$new_file_name);		
		return $new_file_name;
	}
	
}

?>