Simple url shortener in php and sqlite

.htaccess

RewriteEngine on

RewriteCond %{SCRIPT_FILENAME} !-f
RewriteCond %{SCRIPT_FILENAME} !-d
RewriteRule (.*) /index.php?$1

index.php

<?php

header('Content-Type: text/plain');

function hash36($str) {
  $arr = unpack("C*", pack("L", crc32($str)));
  return implode(array_map(function($number) {
    return base_convert($number, 10, 36);
  }, $arr));
}

function self_url() {
  $path = preg_replace("/\?.*$/", "", $_SERVER['REQUEST_URI']);
  $protocol = ((!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] != 'off') || $_SERVER['SERVER_PORT'] == 443) ? "https://" : "http://";
  return $protocol . $_SERVER['HTTP_HOST'] . $path;
}

$filename = "db.sqlite";
$init = !file_exists($filename);
$db = new PDO('sqlite:' . $filename);
//$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
if ($init) {
  $db->query("CREATE TABLE urls(hash VARCHAR(10), url TEXT)");
}

$query = $db->prepare("SELECT * FROM urls WHERE hash = ?");

if (isset($_GET['url'])) {
  $hash = hash36($_GET['url']);
} else {
  $hash = $_SERVER['QUERY_STRING'];
}

if (strlen($hash) > 0 && $query->execute(array($hash))) {
  $result = $query->fetchAll(PDO::FETCH_ASSOC);
  if (count($result) > 0) {
    if (isset($_GET['url'])) {
      echo self_url() . $result[0]['hash'];
    } else {
      header('Location: ' . $result[0]['url'], true, 301);
    }
  } elseif (isset($_GET['url'])) {
    $res = $db->prepare("INSERT INTO urls VALUES(?, ?)");
    if ($res->execute(array($hash, $_GET['url']))) {
      echo self_url() . $hash;
    }
  }
}

?>