Description:
------------
PDOStatement::bindParam mutates it's $variable type to string. This information, white donated in it's declaration (the ampersand), it doesn't mention the side effect that it will actually mutate that variable by converting it to a string.

Please make a notation that the $variable arg will be mutated to a string if it's not already. You should also note that if they wish to avoid this behavior they should use PDOStatement::execute and pass an array to it, as shown below.

Test script:
---------------
    public function getCalls(?DateTime $timeStart = null, ?DateTime $timeEnd = null): float
    {
        $statement = $this->db->prepare(self::SQL_GET_CALLS);
        $statement->bindParam(':timeStart', $timeStart);
        $statement->bindParam(':timeEnd', $timeEnd);
        $statement->execute();
        return (($statement->fetch()['Calls']) ?: 0.0) * 2;
    }

    public function getCalls(?DateTime $timeStart = null, ?DateTime $timeEnd = null): float
    {
        $statement = $this->db->prepare(self::SQL_GET_CALLS);
        $statement->execute([':timeStart' => $timeStart, ':timeEnd' => $timeEnd]);
        return (($statement->fetch()['Calls']) ?: 0.0) * 2;
    }


Expected result:
----------------
For the bindParam to NOT silently change the type of a variable I send it.

Actual result:
--------------
It changes the type of the variable I send it to a string ... How rude.