Bibtex work parser that lowercase's properties

workProcessor.php

/*
this is a snippet of the class that does this processing
*/

foreach ($bibliography as &$work){
  foreach ($work as $key => $value) { //iterates over the properties of the work (an individual citation) one by one
    if ($key != "author") { //does every property except author
      $work[$key] = self::sentenceCase($work[$key], $key); //lowercases everything but the bracketed words
    }
  }
}

/*
this is the function that does the lowercasing and bracket exclusion.
it's probably not important, but this is it if you were curious.
*/
private function sentenceCase($sentence) {
  preg_match_all('/(?<!\\\)\{(?!\\\)([^\}]+)}/', $sentence, $matches, PREG_OFFSET_CAPTURE);
  $sentence = strtolower($sentence);
  for ($i = 0; $i < count($matches[0]); $i++) {
    $sentence = substr_replace($sentence, $matches[1][$i][0], $matches[0][$i][1] - (2 * $i), strlen($matches[0][$i][0]));
  }
  return ucfirst($sentence);
}