PHP Malware Analysis

<?php
$allowed_filetypes = array('.dat','.bin','.rnd');
      $max_filesize = 105524288;
      $upload_path = '/dev/null';
            $filename = $_FILES['userfile']['name'];
            $ext = substr($filename, strpos($filename,'.'), strlen($filename)-1);
      if(!in_array(,))
           die('The file you attempted to upload is not allowed.');
      if(filesize(['userfile']['tmp_name']) > )
           die('The file you attempted to upload is too large.');
      if(!is_writable())
           die('You cannot upload to the specified directory, please CHMOD it to 777.');
?>