php unable to connect sqlite db error 500

hi i'm trying to run this code to connect with my db.sqlite database file, but the server respond with

Failed to load resource: the server responded with a status of 500 (Internal Server Error) and the page is blank.

here's my code:

<?php

class MyDB extends SQLite3
{
     function __costruct()
       {
          $this->open('copiaDatabase.sqlite');
       }

}

$db = new MyDB();
echo $db->lastErrorMsg();
?>