My searching algorithm

search.php

<?
class Dictionary {
  function __construct($filename) {
    $this->filename = $filename;
    $this->sortedFilename = "${filename}.sorted";
  }

  function search($search) {
    if (!file_exists($this->sortedFilename)) {
      $this->sortWords();
    }

    $searchWithComma = ",$search,";

    $size = filesize($this->sortedFilename);
    $file = fopen($this->sortedFilename, 'r');

    $down = 0;
    $up = $size - 1;
    while ($down < $up) {
      $mid = (int)(($down + $up) / 2);
      fseek($file, $mid);
      $line = fread($file, 1024);

      if (strpos($line, $searchWithComma) !== false) {
        fclose($file);
        return true;
      }

      $firstComma = strpos($line, ',');
      $secondComma = strpos($line, ',', $firstComma + 1);
      $word = substr($line, $firstComma + 1, $secondComma - $firstComma - 1);
      if ($search < $word) {
        $up = $mid - 1;
      } else {
        $down = $mid + 1;
      }
    }

    fclose($file);
    return false;
  }

  private function sortWords() {
    $string = file_get_contents($this->filename);
    $pieces = preg_split('/,/', $string);
    sort($pieces);

    $output = fopen($this->sortedFilename, 'w');
    foreach($pieces as $piece) {
      fwrite($output, ',');
      fwrite($output, $piece);
    }
    fwrite($output, ',');

    fclose($output);
  }
}

function searchAndPrint($dictionary, $search) {
  if ($dictionary->search($search)) {
    echo "$search => found\n";
  } else {
    echo "$search => not found\n";
  }
}

$dictionary = new Dictionary('dictionary2.txt');
searchAndPrint($dictionary, 'AAX');
searchAndPrint($dictionary, 'ABNEGATING');
searchAndPrint($dictionary, 'SLAVISHLY');
searchAndPrint($dictionary, 'ZZZZ');
searchAndPrint($dictionary, 'ZZZA');
?>