RarEntry::getCrc() returs a lowercase hex-string (e.g. 'bf6fa85c') the same as hash_... functions, using the same polynomial as 'crc32b' algorithm.
So, it can be used to check CRC after a stream unpacking:

<?php
  $archive_name = 'archive.rar';

  $entry_name = 'someentry.ext';

  $rar = RarArchive::open($archive_name) or die("Cannot open archive $archive_name");

  if ($rar->isBroken()) {
    die("The archive is broken!");
  }

  $entry = $rar->getEntry($entry_name) or die("Cannot find entry $entry_name");

  $stream = $entry->getStream() or die("Cannot open stream");

  $crc = hash_init('crc32b'); // Initializing the hash function

  while (!feof($stream)) {
    $s = fread($stream, 8192);
    if ($s === false) {
        // Error reading (do not use fread(...) or die(...), because fread can return '0'!)
      die('Error reading the compressed file.');
    }
    hash_update($crc, $s); // updating the hash

    // ...
    // Do whatever with the $s
  }

  fclose($stream);

  $got_crc = hash_final($crc);
  $need_crc = $entry->getCrc();

  print("Got CRC: $got_crc" . PHP_EOL);
  print("Need CRC: $need_crc" . PHP_EOL);

  if ($got_crc != $need_crc) {
    // rollback
    print("Sorry guys, the file was incorrect!" . PHP_EOL);
  } else {
    print("Everything is ok" . PHP_EOL);
  }
?>