Detailed explanation of the example of uploading pictures with progress bar using php+ajax

<?php
if(isset($_FILES["FileInput"]) && $_FILES["FileInput"]["error"]== UPLOAD_ERR_OK)
{
  ############ Edit settings ##############
  $UploadDirectory  = &#39;F:/Websites/file_upload/uploads/&#39;; //specify upload directory ends with / (slash)
  ##########################################
  /*
  Note : You will run into errors or blank page if "memory_limit" or "upload_max_filesize" is set to low in "php.ini".
  Open "php.ini" file, and search for "memory_limit" or "upload_max_filesize" limit
  and set them adequately, also check "post_max_size".
  */
  //check if this is an ajax request
  if (!isset($_SERVER[&#39;HTTP_X_REQUESTED_WITH&#39;])){
    die();
  }
  //Is file size is less than allowed size.
  if ($_FILES["FileInput"]["size"] > 5242880) {
    die("File size is too big!");
  }
  //allowed file type Server side check
  switch(strtolower($_FILES[&#39;FileInput&#39;][&#39;type&#39;]))
    {
      //allowed file types
      case &#39;image/png&#39;:
      case &#39;image/gif&#39;:
      case &#39;image/jpeg&#39;:
      case &#39;image/pjpeg&#39;:
      case &#39;text/plain&#39;:
      case &#39;text/html&#39;: //html file
      case &#39;application/x-zip-compressed&#39;:
      case &#39;application/pdf&#39;:
      case &#39;application/msword&#39;:
      case &#39;application/vnd.ms-excel&#39;:
      case &#39;video/mp4&#39;:
        break;
      default:
        die(&#39;Unsupported File!&#39;); //output error
  }
  $File_Name     = strtolower($_FILES[&#39;FileInput&#39;][&#39;name&#39;]);
  $File_Ext      = substr($File_Name, strrpos($File_Name, &#39;.&#39;)); //get file extention
  $Random_Number   = rand(0, 9999999999); //Random number to be added to name.
  $NewFileName    = $Random_Number.$File_Ext; //new file name
  if(move_uploaded_file($_FILES[&#39;FileInput&#39;][&#39;tmp_name&#39;], $UploadDirectory.$NewFileName ))
    {
    die(&#39;Success! File Uploaded.&#39;);
  }else{
    die(&#39;error uploading File!&#39;);
  }
}
else
{
  die(&#39;Something wrong with upload! Is "upload_max_filesize" set correctly?&#39;);
}