PHP function to sanitize the file name to be url safe

filename_sanitizer.php

/*
 * Function to sanitize the file name for url and file name safe.
 *
 * @param string $filename
 *   The unsafe filename is taken as input
 *
 * @return string
 *   The safe file name is returned.
 */
function filename_sanitizer($unsafeFilename){

  // our list of "unsafe characters", add/remove characters if necessary
  $dangerousCharacters = array(" ", '"', "'", "&", "/", "\\", "?", "#");

  // every forbidden character is replaced by an underscore
  $safe_filename = str_replace($dangerousCharacters, '_', $unsafeFilename);

  return $safe_filename;
}

You should also add < and > to the blocklist