Come mai con questo script gli upload non vanno a buon fine?

Codice PHP:
<html>
  <head>
  <title>File multi upload</title>
  </head>
  <body>
<?php
  $UPLOAD_DIR = $_SERVER["DOCUMENT_ROOT"] . "/test/img";
  if(isset($_POST['submit'])){
    $files = array();
    $campo = array('nome','tipo','temp','erro','size');
    $files_info = $_FILES['filename'];
    $successo = ":[/b] Operazione effettuata con successo!</p>";
    $errore = ":[/b] Si è verificato un errore</p>";
    $k = 0;
    foreach ($files_info as $file_row){
      $i=0;
      foreach ($file_row as $data){
        $files[$i]["$campo[$k]"] = $data;
        $i++;
      }
      $k++;
    }
    foreach($files as $file){
      if(($file['erro'] == UPLOAD_ERR_OK)
        and (is_uploaded_file($file['temp']))){
        if ($file["size"] > 0){
          if (move_uploaded_file($file['temp'],
            $UPLOAD_DIR.$file['nome'])){
            echo "

[b]Upload di ".$file['nome'].$successo;
          }
          else{
            echo "

[b]Upload di ".$file['nome'].$errore;
          }
        }
        else {
          echo "

[b]Upload di ".$file['nome'].$errore;
        }
      }
    }
  }
?>
  <h3>Carica un file</h3>
    <form method="post" action="upload.php"
      enctype="multipart/form-data">
      <input type="file" name="filename[]">
      

      <input type="file" name="filename[]">
      

      <input type="file" name="filename[]">
      

      <input type="file" name="filename[]">
      

      <input type="submit" name="submit" value="Upload">
    </form>
  </body>
</html>