File is not being uploaded properly - php

Question

In my android app, I've to upload file to server and read the file at server side. File is being uploaded but some times what happen, in folder (at server in which file resides) if previously uploaded file exists, then the current uploaded file is generated with previous file data. e.g. say if I upload file named A to server with data say '1, 2 and 3' then there will be file A.txt under folder at server side with same data, and if now I upload file named B to server with data say 'A, B and C' then file B.txt under folder will be there but with previous file data of A.txt. So in B.txt, data is '1, 2 and 3'. Below is my code. What can be the issue?
Android Side
public void uploadUserFriendId(String user_id, String filePath, String fileName)
{
String server_url = "http://addressofserver/folder/myfile.php";
InputStream inputStream;
try
{
inputStream = new FileInputStream(new File(filePath));
byte[] data;
try
{
data = IOUtils.toByteArray(inputStream);
HttpClient httpClient = new DefaultHttpClient();
httpClient.getParams().setParameter(CoreProtocolPNames.USER_AGENT,
System.getProperty("http.agent"));
HttpPost httpPost = new HttpPost(server_url);
InputStreamBody inputStreamBody = new InputStreamBody(new ByteArrayInputStream(data), fileName);
MultipartEntity multipartEntity = new MultipartEntity();
multipartEntity.addPart("file", inputStreamBody);
multipartEntity.addPart("user_id", new StringBody(user_id));
httpPost.setEntity(multipartEntity);
HttpResponse httpResponse = httpClient.execute(httpPost);
// Handle response back from script.
if(httpResponse != null) {
//Toast.makeText(getBaseContext(), "Upload Completed. ", 2000).show();
} else { // Error, no response.
//Toast.makeText(getBaseContext(), "Server Error. ", 2000).show();
}
} catch (IOException e) {
e.printStackTrace();
}
} catch (FileNotFoundException e1) {
e1.printStackTrace();
}
}
PHP Side
<?php
error_reporting(~0);
ini_set('display_errors', 1);
mysql_connect("localhost", "root", "Admin#123") or die(mysql_error()) ;
mysql_select_db("retail_menu") or die(mysql_error()) ;
$today =date("YmdHis");
$pic=$today.".txt";
if (isset($_POST["user_id"]) && !empty($_POST["user_id"]))
{
$user_id=$_POST['user_id'];
}
else
{
$user_id="null";
}
$objFile = & $_FILES["file"];
// here file is created under folder named upload at server side
if( move_uploaded_file( $objFile["tmp_name"], "upload/".$user_id.".txt" ) )
{
$file = fopen("upload/".$user_id.".txt" ,"r");
while(! feof($file))
{
$friend_id = fgets($file, 8192);
if($friend_id!= null)
{
$query = "INSERT IGNORE INTO table_name (user_id, friend_id) values('$user_id', '$friend_id')";
var_dump($query);
mysql_query($query);
}
}
fclose($file);
}
else
{
print "There was an error uploading the file, please try again!";
}
?>