<?php
$path = "C:/path/to/textfile"; // <- Do not use trailing slashes in path's
$parts = array(
  'HD ', 'BY ', 'WC ', 'PD ', 'SN ', 'SC ', 'PG ', 'LA ', 'CY ',
  'LP ', 'TD ', 'CO ', 'IN ', 'NS ', 'RE ', 'IPC', 'PUB', 'AN '
);

function extract_parts(array $parts, $str, $eol = PHP_EOL) {
  $ret = array();
  $current_record = array();
  $current_keyword = null;
  foreach (explode($eol, $str) as $line) {
    $line=trim($line);
    $substr = substr($line, 0, 3);
    if ($substr == 'HD ') {
      if (count($current_record)) $ret[] = $current_record;
      $current_record = array_fill_keys($parts, '');
    }
    if (isset($current_record[$substr])) {
      $current_keyword = $substr;
      $line = trim(substr($line, 3));
    }
    if ($current_keyword) $current_record[$current_keyword] .= $line;
  }
  if (count($current_record)) $ret[] = $current_record;
  return $ret;
}

if ($handle = opendir($path)) { // Tip: using glob() would be more comfortable
  while (false !== ($file = readdir($handle))) {
    if ('.' === $file) continue;
    if ('..' === $file) continue;
    $ret = extract_parts($parts, file_get_contents($path . DIRECTORY_SEPARATOR . $file));
  }
}

var_dump($ret);