You didn't escape your database inputs. Always escape!
Here's one way
$sql_query = "UPDATE test_table SET description = '". mysql_real_escape_string($js_encoded). "' WHERE id = 0";
Better yet, use a database wrapper like PDO or ADODb, which would take care of the escaping for you. It would look something like this:
$db->Execute("UPDATE test_table SET description =? where id=?", array($js_encoded, $id));